Welcome to the ODK forum, @sapperdeflap! We're glad you're here. When you get a chance, please introduce yourself on this forum thread. I'd also encourage you to add a picture as your avatar because it helps build community!
The start & end times should be reliable. It uses the device time, so if the date/time on the device is wrong, the submitted times will also be wrong. But that wouldn't cause the issue that you're seeing here.
They may have filled out paper forms during the survey, and then transferred it into the phone app at a later time?
It is possible to enable (prior to data collection) a more detailed audit log. It will provide details on time spent on each question, etc. For correctly setting up the audit feature with KoBo Toolbox you will need to check the KoBo forum and KoBo documentation.