Make of this news story what you will, but apparently Sussex Police ran a bait trap test by enabling mobile phones with GPS tracking bugs and deliberately left them in particular locations in Hastings to see if the mobile phones would be picked up and used by their finders. Not one of the entrapment units were taken, instead it seems they were handed in. It is not clear how thieves have been deterred?
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Just to make it interesting though
Is it clear to you why it would be 'illegal' (forget morals and ethics about found property for the moment) to find a mobile phone and keep it, as opposed to being expected to hand it in? That is apart from the obvious possible crime of using the genuine owner's SIM card inside the phone without the owner's consent eg obtaining a service, namely a telecommunications services, with the intention of avoid payment etc etc. Where does the finder find a constructrive notice that says the phone is registered or something else and that it would be illegal to use the phone? How would the finder actually know who the genuine owner is?
Assuming an honest person had kept the phone but throws away the SIM card. What would be the offence? There is no theft of the phone as it was lost and found. It (the phone) wasn't pinched, stolen (and so on) from an owner. Which statutory provision enforces the rule or duty for a person who finds a mobile phone (not treasure trove) to hand it in?
This form of honeypot trap had have good intention behind implementing it, and in this case it was associated with Sussex Police campaign awareness about stolen phones and immobilisation. This could have rebounded badly if it amounted to an invasion of the finder's privacy or something else.