Against my better judgement, I decided to put her and her brother in the large rack in their 16qt sterilite tubs, which have latches to hold the lids on. The gap above the latch was just large enough in the back for her to squeeze out. I've moved her sibling back on top of the rack with a lid. He'll be staying in a lidded enclosure for the next few months until he's big enough to be contained safely in the rack. We did a full sweep of the room and most of the hallway and downstairs, cleaned out a couple closets, but so far no sign of her. There are tons of places she could have wedged herself into, and at this point she won't be coming out during the day. We'll put out pinkie traps tonight, hopefully she's getting hungry now.
Lesson learned: flat-topped tubs only for babies in racks. I should have waited til they were at least 25g+, maybe even larger.