never, ever seen a cap fall off that was installed correctly. If the cap was left off the 2.5l engine the oil will blow out the hole all over the underside of the hood and engine. the is cap probably on the tech's bench.
if you shut the engine down right away you should be alright. Anything after that is speculation. Does it run quiet now that the oil is topped off? If yes, get them to get a new cap and be on your way; it will be difficult to prove wear (maybe compression test or measure blowby as nissan did when there 2.5l engines had bad rings from the factory ). If it has a permanent knock, go for a new engine.