In addition, document the lost device’s chain of custody as far back as you can trace it. Ask the wireless store employee, parent, school official or other civilian to recall as precisely as possible what they did and how they did it. Ask who else has had access to the device and what they did. Interview them if possible, and also obtain a written witness statement. Put this in your report, as you would after interviewing anyone who entered an unsecured crime scene.
Ideally, preserve evidence on the phone by capturing a forensic image. With the right mobile forensics tool, this is a simple procedure that requires minimal technical expertise; the officer need only provide the image -- together with the device -- to a forensics lab for analysis.
In turn, an in-depth forensic examination will show when criminal evidence was downloaded to or generated on a phone, so the issue would not be whether police will be accused of planting evidence.
Just as for any other piece of evidence, procedural matters such as proper preservation and chain of custody will be the issue. Have an SOP in place, both for typical and less typical seizures. Remember that there is no such thing as too much documentation in cases like these.