diff --git a/balloony.go b/balloony.go
index 1289c73..6ee9b91 100644
--- a/balloony.go
+++ b/balloony.go
@@ -32,6 +32,7 @@ var message_unusual = "Unusual Sonde Detected!"
 
 var receivers []Point
 var receiversMutex sync.RWMutex
+var seenSerials = newSerialTracker()
 
 const defaultReceiversUpdateInterval = 12 * 60 * 60 // 12 hours in seconds
 const zeroWidthSpace = "\u200B"
@@ -64,6 +65,8 @@ var messagePubHandler mqtt.MessageHandler = func(client mqtt.Client, msg mqtt.Me
 
 	// In most situations, we only get 1 packet, but we still handle it with a foreach in the situation where we have a multi-sdr receiver
 	for _, pkt := range pkts {
+		wasSeen := seenSerials.wasSeenRecently(pkt.Serial)
+		seenSerials.markSeen(pkt.Serial, pkt.TimeReceived)
 		// TEST: Test the nearest point functionality
 		// closest, dist, err := FindClosestPoint(pkt.Lat, pkt.Lon, launchSites)
 		// if err != nil {
@@ -75,9 +78,9 @@ var messagePubHandler mqtt.MessageHandler = func(client mqtt.Client, msg mqtt.Me
 		// This is the main processing loop for incoming sondehub packets
 		// Check to see if the sonde is inside of our area of interest
 		if !InsidePoly([]float64{pkt.Lon, pkt.Lat}, boundaryPts) {
-			// Skip packets that are outside the defined boundary
-			if !bypassLocationFilter {
-				return
+			// Skip packets outside the boundary unless we have seen this serial recently.
+			if !bypassLocationFilter && !wasSeen {
+				continue
 			}
 		}
 
@@ -447,8 +450,9 @@ func main() {
 		panic(err)
 	}
 
-	// Start the receivers updater goroutine
+	// Start periodic background workers
 	startReceiversUpdater()
+	startStaleSeenSerialsCleanup(seenSerials)
 
 	// Wait for Ctrl+C (SIGINT) to exit
 	c := make(chan os.Signal, 1)
diff --git a/tracker.go b/tracker.go
new file mode 100644
index 0000000..d82c4f6
--- /dev/null
+++ b/tracker.go
@@ -0,0 +1,65 @@
+package main
+
+import (
+	"sync"
+	"time"
+)
+
+const (
+	staleSeenTTL         = time.Hour
+	staleCleanupInterval = time.Hour
+)
+
+// serialTracker keeps a lightweight in-memory record of recently seen serials.
+type serialTracker struct {
+	mu       sync.RWMutex
+	serials  map[string]struct{}
+	lastSeen map[string]time.Time
+}
+
+func newSerialTracker() *serialTracker {
+	return &serialTracker{
+		serials:  make(map[string]struct{}),
+		lastSeen: make(map[string]time.Time),
+	}
+}
+
+// wasSeenRecently reports whether a serial exists in memory.
+func (t *serialTracker) wasSeenRecently(serial string) bool {
+	t.mu.RLock()
+	_, ok := t.serials[serial]
+	t.mu.RUnlock()
+	return ok
+}
+
+// markSeen updates the serial timestamp to "now".
+func (t *serialTracker) markSeen(serial string, now time.Time) {
+	t.mu.Lock()
+	t.serials[serial] = struct{}{}
+	t.lastSeen[serial] = now
+	t.mu.Unlock()
+}
+
+// removeStale drops serials that have not been seen within maxAge.
+func (t *serialTracker) removeStale(now time.Time, maxAge time.Duration) {
+	cutoff := now.Add(-maxAge)
+	t.mu.Lock()
+	for serial, seenAt := range t.lastSeen {
+		if seenAt.Before(cutoff) {
+			delete(t.lastSeen, serial)
+			delete(t.serials, serial)
+		}
+	}
+	t.mu.Unlock()
+}
+
+func startStaleSeenSerialsCleanup(tracker *serialTracker) {
+	go func() {
+		ticker := time.NewTicker(staleCleanupInterval)
+		defer ticker.Stop()
+
+		for now := range ticker.C {
+			tracker.removeStale(now, staleSeenTTL)
+		}
+	}()
+}