I was terrified I wouldn't be able to find her amidst the piles of stuff in the living room (much of it rescued from the flooded basement months ago), but a moment after I spotted a few random items on the foyer floor, I realized she was still on the ledge next to the door -- her head poking around in midair, perhaps somewhat confused by the random keyring she'd managed to get her neck through. I briefly considered grabbing the camera to take a picture before grabbing her, but thought better of it. She might have taken the opportunity to get away. So, I picked her up without incident or complaint, got the keyring off (she certainly wouldn't have been able to eat with it on), and got her back in her tank.
One of the brand new pieces of tape holding the cover shut was no longer affixed to the side, just the top, so I'm guessing it got peeled off and not properly re-affixed; it couldn't really have just come loose. Luckily, no harm done, unless you count the lost hour of sleep!