In the mishna B"M 1:6, it talks about finding a loan document with a lien on real property ("achrayus"), and not returning it, because we are afraid that the borrower and the lender are in cahoots to defraud a third party ("purchaser") who purchased the encumbered land from the borrower.
How did this actually work in practice? Was there any means for doing a title background check to make sure that land was not mortgaged? Did the purchaser get any documentation about the sale? Why do we care that the document was lost and then found? if the borrower and lender wanted to defraud the purchaser, they could do so even without "losing" the document first.