My dog keeps peeing in the apartment...?

...she is 3, a pug, and housebroken!! That's the worst part! She even knows what the word "potty" means as she'll do it on command, almost, when she is outside. The thing is though, 9 times out of 10 I never catch her in the act. She'll get up in the middle of the night while I'm sleeping - this does not happen ALL the time but it's enough to frustrate me. What do I do?