it's not a bad idea, but it's obviously ripe for just an additional layer of bureaucratic waste. clearly, there is a lack of trust that the police could handle evidence properly. so the natural follow up is who is trust worthy? how would it work? cops are called to the scene, but are then only there to "secure it" until some 3rd party comes along to collect evidence? what's the purpose of a detective at that point? we're not just shifting that role from a police agency to some 3rd party. are we going to re-establish the Pinkertons? if the detectives are no longer a role in the police, then more than likely those that would be attracted to that role would just join the 3rd party instead. so you haven't solved anything other than these people are no longer part of the police but are still involved.