Ah! My UPS wakes me up in the night when I get a power outage - I have 2 in the house - both 2 rooms away in different directions, and I can hear both, thru walls and closed doors. It seems the pitch carries well, even at low volume. But you would have known of a general power outage.

I have found though that occasionally one of the little beasties gets upset for apparently no good reason, no power outage, etc. Just switching it off and turning it off at the wall then back on, and waiting a few minutes before restarting this UPS makes it happy. I have never known mine to reset by itself though.