Probably a deposit for the value of the computer itself if it isn’t returned or something.

I learned this the hard way with my 1974 MG B. I test drove it and looked it over from all angles and there didn’t seem to be any problematic rust. But lo and behold, digging into the floors a little bit they are pretty much rotten and now I’m doing a big restoration project. The lesson I learned the hard way, rust is 10x worse on the inside of the metal than it looks on the outside.