1) Install a safe in the depot.
2) Put all the bus keys/fuel bowser keys/workshop keys/diesel glowpins/tyre valve cores etc etc in the safe
3) withhold the combination until wages paid.
No property was stolen in the making of this bribery action.
As above, but make it into some kind of comedy Anneka Rice Treasure Hunt thing out of it, where your employers or their liquidators have to solve a series of riddles, drive a tractor to Leamington Spa and back, avoid an angry scotsman in a helicopter (or was that The Interceptor) and ford several rivers, only to find all the keys hidden behind some old jazz mags back in the depot staff room. 😀