Unknown AppIndicator Icon Investigation

The "..." icon on the AppIndicator/SystemTray has hover but no click event.

The “…” icon (right) on the AppIndicator/SystemTray has hover effect but clicking on it does nothing.

Some days ago this “…” icon appeared on the AppIndicator/SystemTray, I know that rebooting the computer or restarting the TopBar will probably remove it, but I would like to discover what created it on the first place.

Is there a way to trace/debug which app created this icon?