Hi
I have recently returned from a caving trip in Spain and noticed small blue circular discs a few mm thick attached near the cave number plates.
Does anyone know what they are? Are they some sort of smart marker? I have attached a photo

They look like RF id tags, scan them with compatible devices (most smart phones) and they share a small amount of data.

Purely speculation but it could be the website link for a survey or visitors book? Would be nice to know exactly how they are been used 

Thanks for the info - very interesting