Pho es need a Burndown mode by default. Enable it, and it will continue to operate normally for 6 hours or so, during which time you can disable Burndown. But if you don't, it automatically wipes your data.
No doubt it would be illegal and no manuf would ever actually do it, because it could be construed as destroying evidence. But an automatic Burndown would give you the plausible deniability that you just happen to have a new secondhand phone which hasn't yet been set up.
