This whole thread was very much on my mind the other day when I got "surprised" in the shower (stall, not part of the bathtub). Thankfully it was the hubster home before I expected. The doors were locked, and while the alarm was not armed, it chimes whenever a window opens or a door opens, and that usually is enough to tell me someone has entered. Dogs were inside and on duty.
And I knew he was there when he walked into the bathroom. Not before. The noise of water running drowns out everything short of an alarm system, apparently. I don't know if the dogs barked, never heard the chime, and apparently didn't even here his usual call of "hello". Had he been a bad guy, I'd have been sunk.
So the moral of the story is: take a bath instead.
Springmom