Souce can only be timed out if timeout is non-zero.

types.go

@@ -18,7 +18,8 @@ type Source struct {
 }
 
 func (s Source) TimedOut() bool {
-	return time.Since(s.LastSeenAt) > time.Duration(s.AlertTimeout)*time.Second
+	return s.AlertTimeout != 0 &&
+		time.Since(s.LastSeenAt) > time.Duration(s.AlertTimeout)*time.Second
 }
 
 func (s Source) RequiresAlertForTimeout() bool {