I feel there's a bit more to the story of the phone being turned into DQ--just a gut feeling I have. Especially because in this day & age, the chances of a random person turning in a 'found' cell phone are quite slim--in my opinion!
RSBM
At least twice (that I can remember) I have found a cell phone in a public place (once in a pharmacy restroom, once in an airport restroom) and both times I turned it in (to the pharmacy employees and to the nearest gate agent, respectively.)
As Lilibet pointed out, if the guy who turned it in is involved in the MP's disappearance, he's not the brightest crayon in the box. I think it's far more likely that it was found on DQ premises and the man was a good Samaritan just doing the right thing.
The police are probably very interested in finding out from him where exactly on the premises the phone was found, though. Its specific location could be a hint as to whether it was deliberately tossed out as a breadcrumb by the MP or if it was dropped in a struggle. And possibly also to take his fingerprints so they can know which of any prints that might be found on the phone are his.