Can you post a photo of the enclosure and area?
I would go back and check around the tank, it probably went into the nearest little space or drawer or bookshelf it could get into and balled up. It helps to use a flashlight when searching. They don't like open areas so it would have moved along the walls.
It's likely to be under or in something. So you might be looking somewhere from above and think it can't possibly be underneath that box/book/bag but don't assume, lift up everything and check underneath and inside. And behind bookcases even if you think they're right up against the wall and there's no space behind. Also behind the books/dvds on a bookshelf, even if there doesn't appear to be much space behind them.