OK the large component is a piezo buzzer - I think it is - (PKB6-5A0)
Is there a connection from the other side of the rheostat? I can't see one.
There is also a small Motorolla component which I can't identify.
Anyway FWIW - holding tank alarm seems very likely based on your observations of where the wire goes. The button is probably a temporary reset of the alarm.
Take a close look at the red wire from the rheostat. It appears very close to shorting out against the fuse terminal...