You are one perseptive mom and deserve a pat on the back.
My daughter (now 30) did this in High School. She and her friends would go to a school dance and have a "Slumber Party" at one of the girls' home. I found out much later (like when she was in her twenties) that these Slumber Parties included boys and that they partied all night. I can't believe I was that naive.
Be tough, give her a curfew and get that girl home!