Could you not just say something like 'What? I left them all on my desk, so someones clearly moved them?' or something.
Then the onus is on them to prove you never and they'll surely not be arsed to?
you reminded me of something I did when I left some place
I worked in a section for 10 years, usually did a collection for someone who left, bit of tea and cake etc.
Nothing was done for me, literally no effort made whatsoever which I thought was poor given all the people I looked after and all the people I had trained in down the years. To make matters worse the absolute weasel of a boss sent someone to tell me I was barred from their canteen (instead of coming to me himself he sat in his office and sent someone else to do it like the spineless c*nt he was).
Anyway our office was still unmanned so i went in one day, locked all their desk drawers, locked all their cabinets and said nothing. Few weeks later they had difficulty moving someone in.
came to me
"what happened all them keys"
"havent a clue, havent been there in ages"
I had them all along, took them home and chucked them in the bin. They had to get a load of new ones cut. Hope it cost a few bob