After a car accident I lost my phone for over 4 months while they did whatever they do to prove I wasn't using a phone or had deleted anything to hide if I had been on the phone. I'm sure they also took all the x rated photos off my phone of the young lady that worked in the stores department at work. I actually figure that had more to do with the time it took to get my phone back.
seems a bit inefficient - I mean providing they have access to the phone presumably all they need do is take an image of it using whatever official evidence gathering software/hard wear they use then hand it back
they can then check messaging apps, calls etc.. and contact the network to check on SMS activity, calls etc.. too