Was it actually stolen though? Did a researcher take it to their office, mess up the paperwork to remove it, and was hit by a bus before they could return it.
Do we even know when it went missing? It was discovered missing two decades ago, but was it in fact missing for longer but noone noticed?
I can imagine someone finding it in an old cupboard when an old don died and being embarrassed that they didn't return it immediately, so they did this
The article mentions that they were taken out for photography in late 2000 and later noticed to be missing in 2001. That doesn’t make your idea impossible, but does mean we kind of know when they went missing.
Do we even know when it went missing? It was discovered missing two decades ago, but was it in fact missing for longer but noone noticed?
I can imagine someone finding it in an old cupboard when an old don died and being embarrassed that they didn't return it immediately, so they did this