Luckily, despite the fact that she probably could fit under the door (Jeff says hamsters can scrunch pretty good), she was still in the office (so why was Copper yowling? Concidence, change in smells in the house?) We chased her around for a few minutes and finally got her back in her cage. We traced the problem back to a door on a space needle-like attachment on the cage. It was ajar. We're not sure if Jeff left it ajar earlier or if she popped it open, but there's a heavy measuring tape hanging off of it as a temporary fix (the other doors are twist-tied or otherwise pinned shut.) Whether or not she opened it, Lucy definitely expected the door to still be ajar - we just watched her try to get back out again, be unsuccessful, and get very pissed.
Oy. I should try to go back to bed.