It's the kind of feature that would rub people the wrong way, whether it's a cop, a friend, or a stranger you want to show a funny video to.

To give a couple of examples, using this would be a bit like locking the glove box in front of traffic cops, or going around the house locking rooms when you have a guest.

The only way to make it socially acceptable would be to make it some sort of default (for example, requiring face id when switching apps, with pin/fingerprint as a backup), which will probably be an option as the lag and failure rate of face id or other owner-detection technologies improve.