The morning girl in kitchen scenario

This morning I went down to the kitchen to fill my thermos up with coffee and there was a girl there. Hmmm. And then she sat down in the living room and didn't say anything. One of my roomie's has a girlfriend so I called out, "Hey are you so and so's girlfriend?" Which she responded, " I'm his ex-girlfriend." I know he wouldn't cheat on his current girl due to our first gossip conversation about how mad he was at his mate for breaking one of the commandments. So if this girl was current girlfriend turned ex why was she still in the vacinity when he wasn't present?