Alternative that seems at least equally likely to me: a student took them, either as a stupid dare/prank or with the thought that they could sell them; then rea

Alternative that seems at least equally likely to me: a student took them, either as a stupid dare/prank or with the thought that they could sell them; then realised they had no idea how to go about fencing such an item, and stashed them away; now, 20+ years later, they've finally grown-up enough to decide that they're never going to make a fortune selling them, and should do the decent thing by returning them unharmed. If that's the case, the cling-film may be 20 years old, which is probably something analysis can establish. Or, the academic who took them eventually died; in sorting through their stuff, their heirs realised what this was and wanted to return them, but didn't want anyone's name dragged into it. In that scenario, the cling-film is more likely recent, just used to package them for returning.