Well, there weren't any animals in the room besides my other rodents and my fish. And my crab. But nothing that would make any noise or threaten her. It's possible that she could have heard something outside that I couldn't. Or she could be in heat. She wasn't wiggling her ears though.
And don't worry guys, she has plenty of company. I guess I should have mentioned it, but I have 17 other mice. She IS currently separated because she's being treated for mites, but under normal circumstances she has 6 other tankmates for company. (And I've been spending as much time as I can with her until she's clear to go back with the others.)