I had a similar story.  But the girlfriend wanted to go into a house my friends dad was doing the tile work on.  We didn't trip off an alarm because I had the codes because I worked for him sometimes.  I did feel like an asshole though...but what guy is gonna turn down their girl when she's offering such a crazy request?? And besides...deep down every guy fears that each time he has sex it could be the last time.  He could get hit by a car... wang could fall off....2012.... you never refuse cause you just never know.  So we sex it up and leave.  
Fast forward a couple of years and I'm hanging out with my friend and his dad.  They go into this story about how his dad lost the contract on this guys houses because the owner got paranoid and installed cameras and caught two people banging in there.  They blamed my friends dad and said he never put the alarm back on or locked up after he left.  So now I feel absolutely awful.  Finally his dad leaves, and I lean over and tell my friend that it was me.  He tells me we should tell his dad so when he gets back I did.  They said it was cool but I could tell they were pretty pissed.  Finally after a couple hours they start to crack up.  I forgot I had told my friend what I did so him and his dad made up the story to fuck with me.  So there that is.