If you are under surveillance it's likely they would have observed this already. Don't assume they would go beyond removing all of your valuables and clothes right then and there if they thought this setup was in place.

Some of the above methods can still be broken. Requiring a face present? Why not just print a big blowup photo and quickly insert in front of camera?

There are smart people getting around security.

That's not a bad idea. You could use some kind of USB dongle connected to a wearable? Once you walk away from computer it takes a picture of the screen( looks like nothing is changed), but the computer immediately locks down everything except what's currently on the screen. Only a password along with the wearable would unlock it? (It's probally not easily feasible, but it doesn't sound impossible?) "Oh, I scratched my head, and my computer hard drive shut down."