diff --git a/src/zm_zone.cpp b/src/zm_zone.cpp
index bff7dff09..4715f7a9e 100644
--- a/src/zm_zone.cpp
+++ b/src/zm_zone.cpp
@@ -984,54 +984,20 @@ std::vector<Zone> Zone::Load(const std::shared_ptr<Monitor> &monitor) {
     /* HTML colour code is actually BGR in memory, we want RGB */
     AlarmRGB = rgb_convert(AlarmRGB, ZM_SUBPIX_ORDER_BGR);
 
+    // Auto-detect coordinate format: decimal points mean percentages,
+    // integer-only means legacy pixel values. Units field is not trusted.
     Debug(5, "Parsing polygon %s (Units=%s)", Coords, Units);
     Polygon polygon;
-    if (!strcmp(Units, "Pixels")) {
-      // Legacy pixel-based coordinates: parse as integer pixel values
-      if (!ParsePolygonString(Coords, polygon)) {
+    if (strchr(Coords, '.')) {
+      // Decimal values present — treat as percentages regardless of Units field
+      if (!ParsePercentagePolygon(Coords, monitor->Width(), monitor->Height(), polygon)) {
         Error("Unable to parse polygon string '%s' for zone %d/%s for monitor %s, ignoring",
               Coords, Id, Name, monitor->Name());
         continue;
       }
     } else {
-      // Percentage-based coordinates (default): convert to pixels using monitor dimensions.
-      // However, if any coordinate value exceeds 100, these are actually pixel values
-      // stored with incorrect Units — fall back to pixel parsing with a warning.
-      bool has_pixel_values = false;
-      {
-        const char *s = Coords;
-        while (*s != '\0') {
-          double val = strtod(s, nullptr);
-          if (val > 100.0) {
-            has_pixel_values = true;
-            break;
-          }
-          // Skip to next number: find comma then space (x,y pairs separated by spaces)
-          const char *comma = strchr(s, ',');
-          if (!comma) break;
-          val = strtod(comma + 1, nullptr);
-          if (val > 100.0) {
-            has_pixel_values = true;
-            break;
-          }
-          const char *space = strchr(comma + 1, ' ');
-          if (space) {
-            s = space + 1;
-          } else {
-            break;
-          }
-        }
-      }
-
-      if (has_pixel_values) {
-        Debug(1, "Zone %d/%s has Units=Percent but Coords contain pixel values (>100), "
-                "parsing as pixels instead", Id, Name);
-        if (!ParsePolygonString(Coords, polygon)) {
-          Error("Unable to parse polygon string '%s' for zone %d/%s for monitor %s, ignoring",
-                Coords, Id, Name, monitor->Name());
-          continue;
-        }
-      } else if (!ParsePercentagePolygon(Coords, monitor->Width(), monitor->Height(), polygon)) {
+      // Integer-only coordinates — treat as pixel values
+      if (!ParsePolygonString(Coords, polygon)) {
         Error("Unable to parse polygon string '%s' for zone %d/%s for monitor %s, ignoring",
               Coords, Id, Name, monitor->Name());
         continue;
diff --git a/tests/zm_zone.cpp b/tests/zm_zone.cpp
index ac8fc3cf6..a56d0cd45 100644
--- a/tests/zm_zone.cpp
+++ b/tests/zm_zone.cpp
@@ -220,3 +220,106 @@ TEST_CASE("Zone: pixel values through ParsePercentagePolygon produce wrong resul
   REQUIRE(verts[1] == Vector2(static_cast<int>(width), 0));
   REQUIRE(verts[2] == Vector2(static_cast<int>(width), static_cast<int>(height)));
 }
+
+// --- Auto-detect format tests ---
+// The zone loader uses strchr(Coords, '.') to decide format:
+//   decimal point present -> ParsePercentagePolygon
+//   no decimal point      -> ParsePolygonString (legacy pixels)
+// These tests verify both parsers handle the inputs they'll receive
+// under auto-detection, and document the broken case that auto-detection prevents.
+
+TEST_CASE("Zone: ParsePolygonString truncates decimal coords via atoi", "[Zone]") {
+  // This is the bug that broke motion detection: percentage coords like
+  // "0.00,0.00 99.96,0.00 99.96,99.93 0.00,99.93" parsed by
+  // ParsePolygonString (which uses atoi) get truncated to a 99x99 pixel zone
+  Polygon polygon;
+  bool ok = Zone::ParsePolygonString("0.00,0.00 99.96,0.00 99.96,99.93 0.00,99.93", polygon);
+  REQUIRE(ok);
+
+  auto const &verts = polygon.GetVertices();
+  REQUIRE(verts.size() == 4);
+  // atoi("99.96") = 99, atoi("99.93") = 99
+  // On a 2560x1440 monitor this would be a 99x99 pixel zone — essentially no coverage
+  REQUIRE(verts[1] == Vector2(99, 0));
+  REQUIRE(verts[2] == Vector2(99, 99));
+}
+
+TEST_CASE("Zone: decimal coords through ParsePercentagePolygon give correct pixels", "[Zone]") {
+  // Same coords as above, but correctly routed to ParsePercentagePolygon
+  // by the auto-detect logic (decimal point present)
+  Polygon polygon;
+  unsigned int width = 2560;
+  unsigned int height = 1440;
+
+  bool ok = Zone::ParsePercentagePolygon(
+      "0.00,0.00 99.96,0.00 99.96,99.93 0.00,99.93", width, height, polygon);
+  REQUIRE(ok);
+
+  auto const &verts = polygon.GetVertices();
+  REQUIRE(verts.size() == 4);
+  // 99.96% of 2560 = 2558.976 -> 2559
+  REQUIRE(verts[1].x_ == 2559);
+  REQUIRE(verts[1].y_ == 0);
+  // 99.93% of 1440 = 1438.992 -> 1439
+  REQUIRE(verts[2].x_ == 2559);
+  REQUIRE(verts[2].y_ == 1439);
+}
+
+TEST_CASE("Zone: integer pixel coords stay as pixels", "[Zone]") {
+  // Legacy integer-only coords should be parsed as raw pixel values
+  // Auto-detect: no decimal point -> ParsePolygonString
+  Polygon polygon;
+  bool ok = Zone::ParsePolygonString("0,0 2559,0 2559,1439 0,1439", polygon);
+  REQUIRE(ok);
+
+  auto const &verts = polygon.GetVertices();
+  REQUIRE(verts.size() == 4);
+  REQUIRE(verts[0] == Vector2(0, 0));
+  REQUIRE(verts[1] == Vector2(2559, 0));
+  REQUIRE(verts[2] == Vector2(2559, 1439));
+  REQUIRE(verts[3] == Vector2(0, 1439));
+}
+
+TEST_CASE("Zone: auto-detect heuristic — strchr for decimal point", "[Zone]") {
+  // Verify the heuristic used by the zone loader:
+  // strchr(coords, '.') distinguishes percentage from pixel coords
+
+  // Percentage coords always have decimal points from round(..., 2)
+  const char *pct_coords = "0.00,0.00 99.96,0.00 99.96,99.93 0.00,99.93";
+  REQUIRE(strchr(pct_coords, '.') != nullptr);
+
+  // Legacy pixel coords are always integers
+  const char *px_coords = "0,0 2559,0 2559,1439 0,1439";
+  REQUIRE(strchr(px_coords, '.') == nullptr);
+
+  // Edge case: small pixel zone that looks like it could be percentages
+  // but has no decimal points — correctly detected as pixels
+  const char *small_px = "0,0 50,0 50,50 0,50";
+  REQUIRE(strchr(small_px, '.') == nullptr);
+}
+
+TEST_CASE("Zone: percentage coords at various resolutions", "[Zone]") {
+  Polygon polygon;
+
+  // The same percentage zone should produce proportional pixel coords
+  // regardless of monitor resolution
+  const char *coords = "10.00,20.00 90.00,20.00 90.00,80.00 10.00,80.00";
+
+  SECTION("640x480") {
+    bool ok = Zone::ParsePercentagePolygon(coords, 640, 480, polygon);
+    REQUIRE(ok);
+    auto const &v = polygon.GetVertices();
+    REQUIRE(v[0] == Vector2(64, 96));    // 10% of 640, 20% of 480
+    REQUIRE(v[1] == Vector2(576, 96));   // 90% of 640
+    REQUIRE(v[2] == Vector2(576, 384));  // 80% of 480
+  }
+
+  SECTION("3840x2160 (4K)") {
+    bool ok = Zone::ParsePercentagePolygon(coords, 3840, 2160, polygon);
+    REQUIRE(ok);
+    auto const &v = polygon.GetVertices();
+    REQUIRE(v[0] == Vector2(384, 432));   // 10% of 3840, 20% of 2160
+    REQUIRE(v[1] == Vector2(3456, 432));  // 90% of 3840
+    REQUIRE(v[2] == Vector2(3456, 1728)); // 80% of 2160
+  }
+}
diff --git a/web/includes/functions.php b/web/includes/functions.php
index b5dbc08a9..7b6b06df1 100644
--- a/web/includes/functions.php
+++ b/web/includes/functions.php
@@ -1467,7 +1467,7 @@ function limitPoints(&$points, $min_x, $min_y, $max_x, $max_y) {
 } // end function limitPoints( $points, $min_x, $min_y, $max_x, $max_y )
 
 function convertPixelPointsToPercent(&$points, $width, $height) {
-  if (!$width || !$height) return;
+  if (!$width || !$height) return false;
   $isPixel = false;
   foreach ($points as $point) {
     if ($point['x'] > 100 || $point['y'] > 100) {
@@ -1482,6 +1482,7 @@ function convertPixelPointsToPercent(&$points, $width, $height) {
     }
     unset($point);
   }
+  return $isPixel;
 }
 
 function scalePoints(&$points, $scale) {
diff --git a/web/skins/classic/views/zone.php b/web/skins/classic/views/zone.php
index 112b4b767..fd4af1a3a 100644
--- a/web/skins/classic/views/zone.php
+++ b/web/skins/classic/views/zone.php
@@ -96,6 +96,8 @@ if ( !isset($zone) ) {
 
 # Auto-detect pixel coordinates and convert to percentages
 convertPixelPointsToPercent($zone['Points'], $monitor->ViewWidth(), $monitor->ViewHeight());
+# Always store as percentages
+$zone['Units'] = 'Percent';
 
 # Ensure Zone fits within the limits of the Monitor
 limitPoints($zone['Points'], $minX, $minY, $maxX, $maxY);