Just the fact how she left abruptly without any hesitation pretty much tells me she has another guy she is messing with. Most females can't be alone and a lot of them want entertainment nonstop. Once she has her emotional prime, aka new guy she focuses the bulk of her emotional attention on, the other guys become expendable and only exist for some tertiary purpose, whether it be to fix her computer, to cuddle with, to smoke with, or whatever free benefits this man will hand out. She might have gotten done up at the party and that guy left so she needed to find a guy to 'cool down' and possibly cuddle with for the rest of the night since he left. These women know what they are doing. Any woman that goes into a man's house at 3 in the morning knows what's up and what's the general expectation.
I bet you as soon as the current guy she's dealing with slides her to the left, she's going to be right back at your doorstep trying to act all friendly, talking about, "I really missed you! When are we going to hang out again?"