A stranger in my new bathroom

I arrived home a bit late. It was eleven in the evening, but my flatmate had not come back from her party yet. I was extremely tired so I decided to take a shower and go straight to bed. I entered the bathroom and put my things on the edge of the bath. Then I noticed something strange. In the corner of my new bathroom there was a pair of slippers. They did not belong to me, I was sure of that, and they did not belong to my flatmate either. She has small feet and those slippers were large, number nine at least! Well, the slippers belonged to a man, I thought. But there were no guys living in the flat. Then I noticed some other items that only confirmed my theory: an unknown guy was living in my flat and using my new bathroom. Of course, I wouldn"t mind if I knew him but Mary hadn"t introduced anyone to me. At least I couldn"t remember any situations like that. On the shelf next to mine there was a bottle of shaving foam and a toothbrush. There was no doubt that Mary had invited somebody. When I left my new bathroom, I heard the front door open. Two people entered the flat. Then I realised that it was my flatmate and her boyfriend. I had actually met him on the first day, when Mary was moving in, and I had promised to put him up from time to time.