I’m assuming that you’re asking about a piece of furniture as opposed to a concertina. You could put a dab on the under side where it would last indefinitely. Or on the inside of a leg or you could do both.As for a concertina I would put a dab between the bellow folds on the underside.
You register your vial with the smart water company. Where there are two dabs if the original owner claims that the concertina was stolen from them, it would be returned to them, even though you as the second person had bought it in good faith. That’s the law and why they always say buyer beware. If you buy a stolen item even in good faith it’s not legally yours if the original owner wants it back.
There are two types of smart water, ones that work in shops that effectively detonate all over a thief. This is where you see that it’s all through their hair and clothing. The other is sold in tiny vials, often given away by insurance companies if you take out their insurance. I can only describe it as looking like nail varnish but one that goes invisible.
As I said earlier it’s detected by UV, but as someone else pointed out, that’s only the beginning there are markers in there that are invisible. Smart water don’t divulge what they put in it but each vial is unique. You make it yours by registering your details against your vial.
I understand about what people are saying about knowing their concertinas but it does no harm if you’re given a free vial to put a dab on. You’re instrument could be missing for years and go through a pawn shop or an auction house. Most pawn shops check their items as far as I’m aware and a reputable auction house selling a high end instrument should check. None of these places want a reputation of handling stolen goods.