There are some great threads on this so might be worth having a search. Luckily for me I've never had this problem. I think the key is locking everything away, putting up a discreet camera for a short period of time if need be and clearly naming everything.
I've just had a thought though. How about painting your shelf bright red? Get some really slow drying paint and see who has red hands - you'll literally be catching them red-handed (I take no credit for this idea, I remember reading an Enid Blyton story as a child with a similar idea)
Oh and obviously let your YO know that you will repaint it white when you've caught the thief.
Everything has mine or H's name emblazoned all over it! I even had name-tags made (like you used to have for school, but MUCH bigger!) with his name for rugs, head collars etc. It does seem to have worked!
Also, I keep most grooming things, lunge etc in the back of my car (along with a selection of coats for different weathers, woolly hat, waterproof hat, hi-viz bits etc etc)!! Only things left at the yard are tack, rug he is in and one set of head collar/lead rope.
As mentioned on the previous thread, I have;
-left dead rodents where they will be grabbed
- plastered stuff in shoe polish/ gloss paint etc
- left rude notes in stuff i.e if you are reading this then go **** yourself you sticky fingered ****
- removed wheelbarrow tyres, brush heads etc.
- locked stuff up if possible.
- informed everyone my horses have lice
- filled in an order form for robinsons with the regularly borrowed items & stuck it to notice board
- and goes without saying if I catch them I share my thoughts in an eloquent & highly offensive manner.
Remember Max the bouncer's car alarm on "Phoenix Nights" with Peter Kay? It was set to shout "get back you b*****d - I'll break your legs"! That'd make 'em jump!