I think if you look at this in an agile way then you accept that you can’t know everything up front and create your test plan to cover what you know, but allow for change.
The ‘off piste’ testing allows a bit of exploration and should be used to sanity check your test plan decisions. If it exposes a problematic area then great – adapt the plan to cope with it.
You could call ‘off piste’ ‘testing or verifying the test plan’. If it doesn’t find anything then your test plan was good. If it does, enhance the plan.