I've had the timer go off randomly without ever having set it. There have been times where it will show that a 25 minute timer ended while it was charging for over 2 hours, so I never would have been able to set it myself. I also never set the timer on my phone.
I thought that maybe it was a case where the timer was never silenced on my watch when I had originally set it on my phone, but these timers are usually going off hours after I happened to set a timer on my phone.
