i was helping a friend's uncle who was a lawyer once, he wouldn't trust me with his machine to resolve a boot up problem. another word, the friend trust me, but the uncle doesn't. so best thing to do is to find someone that "you" trust to work on your computer. if the tech/thief wants to get your data, he/she will.
once you close a document, a temporary file tthat's associated to the original file is also deleted. where does it go??? that i don't know. to really wipe out a drive, you'll need to format it several times, so that mean your single file that you delete/move to flash drive, still has traces on the original drive. make sense? uhh..i'm not sure, i'm asking myself that.
but overall, if you have confidnetial sensitive document, don't trust franchise or large companies without signing a contract.