I think my roommate is using my toothbrush

I had to buy a new toothbrush because mine disappeared. One of my roommates leaves and sleeps at her boyfriend's place, taking her toothbrush with. One day last week I went to brush my teeth and saw two brushes that weren't mine. I went out a bought a new one, it's blue. My old one did return home but I sure wasn't going to use it. I was going to use it for cleaning the cracks in the shower, so I left it till I felt so inclinded to do so. If we do the math their should be a total of 4 toothbrushes. Two are mine and two, one for each of my roommates. So last night after the horrible Packer I went to clean my pretty teeth. How many toothbrushes? TWO! Two toothbrushes and both are mine, where did they go and why is my old one wet??? What's next, sharing underwear?