I think I meet 'dirty thing'

I was early to college one day and decided to hang out at the library. In the middle of doing something there, I had a call of the nature and went to the ladies. I went into the second cubic which is the middle one. When I entered, I notice that the entire wash room was empty. But when I close the cubic door, and did what I was supposed to do, I heard a girl's voice singing. Quite clear actually and few seconds later I heard the sound of a toilet flushing. I started to freak out when I realized I was supposed to be the only one in the toilet. There was no sign no of the next cubic door opening and no shadow can be seen on the floor. Then I heard someone on the tap, I suppose the 'one' is washing hands and about to leave, so I waited, but the singing and the water sound wasn't stopping, so wth.. I open the cubic door and saw ... NOTHING!!! My gosh. All the voices I heard just now were gone too. I didn't want to think further anymore, just washed my hands and ran out of the washroom asap. I thought that would probably some other girl who just finished using the toilet, but there was no one walking out from the ladies at the hallway. Hmm... Not to scare anyone, but I just feeling like blogging this tonight cause things like this don't happen everyday.