If you don't have any land dwelling pets that might have a lil' shrimp snack off the floor I'd widen my search around the tank. They don't die the instant they leave the water, ever see how much a fish spaz-es around when pulled from the water? It could end up behind the stand or a few feet away from the tank even (but hopefully not). I have no experience keeping that specific tetra or shrimp so can't say if it ended up as a snack or not. Hopefully its being a super ninja behind a plant or the filter. If the shrimp was so big compared to the tetra and they've not been eating when you feed.. do they have 'full bellies' since the shrimp vanished or still looking a bit scrawny?
added: I'd check the larger tank next to the 1g too. See if he managed to fly into there "UPGRADE!"
added: I'd check the larger tank next to the 1g too. See if he managed to fly into there "UPGRADE!"