Bruce Schneier has an amazing piece about something called ‘smart water’:

No, really. It’s liquid with a unique identifier that is linked to a particular owner.

Forensic Coding combined with microdot technology.

SmartWater has been designed to protect household property and motor vehicles. Each bottle of SmartWater solution contains a unique forensic code, which is assigned to a household or vehicle.

An additional feature of SmartWater Instant is the inclusion tiny micro-dot particles which enable Police to quickly identify the true owner of the property.

The idea is for me to paint this stuff on my valuables as proof of ownership. I think a better idea would be for me to paint it on your valuables, and then call the police.