If it was me and I was 100% sure and I mean 100%, totally without a doubt etc etc, I would phone and go along the lines of you have a cam and they have 24 hours to return the item or the police will be shown the recording, I would be surprised if it did not reappear.
Slightest doubt at all and you can't risk accusing a possible innocent person.
You could always ask them if their driver remembered seeing a toolkit, as it's gone missing and you're trying to establish roughly when it disappeared.
You're not accusing him, but if other people have complained about this driver they might put 2 & 2 together and have a word with him.
Watch anyone who comes in the shop like a hawk, better still don't let them even look inside meet them on the yard before they get out the truck.
With certain types I make it clear they shouldn't get out the vehicle if I am not about because of the dogs
best way set a recording camera up find out for sure who it is and then go from there its much better than being accused when it isnt you thats doing it
use a hidden cam just incase its someone else tell no one about it best way to find out
anoying thing is it was at home, the workshop itself is secure as hell. trouble is i'm not 100% sure without dought because i would have had to catch them in the act, and that wouldnt be good i've seen enough horror films and have some nasty machenary