I had a doorbell sigil put down a few days ago with no extending or anything, it disappeared and is invisible to everyone now but it still rings and gives feedback to both the person who steps on it and it rings for the person it's connected to as if it's still there.

1. Have a doorbell sigil put down and connected to someone

2. Wait a few days 

3. The sigil becomes invisible to players, but still rings when someone steps on it and acts as normal.