If I found 30,000 Bt in the street and there was no way of finding out who lost it, I would take it home, put it in an envelope and in my safe and keep my ear to the ground, and if I found anything, I would try and locate the person who would need to say where they lost it and roughly how much they lost.
If it was back in the UK I would take the money to the nearest police station and hand it in, but not to a Thai police station for obvious reasons.