I see nothing whatever wrong with saying The washing machine just signalled off, though ...signalled the end of its cycle, may be better.
I have never heard anyone use "signal" with an appliance, but I see nothing wrong with doing so. It provides clarity as to meaning, and I can't think of any other obvious way of saying it, unless it is something like The washing machine just made an off sound - but to my mind that's a bit clunky.
Other answers suggest things like "the machine emits a noise when it's finished" as alternatives - but that's a different thing. That is a description as to what happens. What I believe the OP is looking for is a way of communicating to someone that the the all-important beep has been heard.