"That jar has now gone missing..."

I must admit, that's the one point that I have great sympathy for metal detectorists about. There's hardly one of them that hasn't had stuff lost that way. Old donations I can understand but there's really no excuse for losing modern ones. You just give them a reference number and a shelf number and you don't move them without noting it on the computer. Tescos manage.
There's a huge wind of change blowing through the museum sector, Renaissance in the Regions etc. I don't think chaotic C19th record keeping will be tolerated much longer. (A retired Admiral is now in charge and he's hot stuff).

Don't worry about Shaun. He'll be a celebrity and have lots of offers to produce exhibition pieces - and his stuff will now sell for oodles.