I'm a giant idiot
I'll preface this story by reiterating that I DID catch a non-mom using our mother's room for personal phone calls earlier this week.
So, I headed to the Mother's room this morning at my scheduled time only to find the door shut. It was booked before me, so I just hung out to wait for the other mom to finish. We're all very good about sticking to our scheduled times, so it was weird that after 10 minutes, the door was still shut. I IM-ed the mom who said she was working from home today.
I knocked on the door & I SWEAR I heard someone inside say 'just a minute.' Now, I've lost half my pumping time & I'm getting p*ssed. I flag down our Facilities manager & go off on a rant about people using the room inappropriately. He knocked but didn't hear anyone inside. So, he went off to get one of the women from the Facilities team. Of course, I haven't tried to actually open the door since the last thing I want to see if one of my co-workers' boobs or worse! 
A female colleague comes over, knocks a couple times, then slowly opens the door to... an empty room. *facepalm* I'm assuming that the cleaning crew closed the door after they straightened up.
So, I lost an extra 30 minutes of my shift that I'll have to explain to my manager, plus looked like a complete lunatic to 2 members of our Facilities team.
I shouldn't be allowed out in public.
Re: I'm a giant idiot
I wish I could console myself with that explanation, but the manager happened to walk by while I was waiting outside the room. I never left the door.
I could have heard someone in a neighboring room say that, though. Either way, the Facilities team is having a good laugh at my expense today. :P