"There is a difference between supervision and spying, though. "
Much will depend on how the data is used too. If the goal is to improve systemic efficiency and not as the means to surveil an individual employee, no problem.
I wonder if holding an employee for "security checks" can be considered kidnapping in some cases. They aren't free to leave and rather than being physically restrained, they are being financially restrained. I imagine that if they don't submit to the security checks, they will be sacked. I've seen some places where the exit is a one way revolving gate and the only other way out would be through a fire-door. Is that physical restraint?
It should be hard to screen people. Scotty at Strange Parts (youtube) did a show in China of a small shop where everything had an RFID tag and you carded in and the system charged you for all of the items you took. He went on to research the tags and it's well worth watching. The tags are super cheap. They have to be if they are going to be used on a packet of crisps. The same sort of thing could be used to scan employees on the way in/out as the system would tell security what the item is. Somebody like Amazon could require vendors to include the tags inside the packaging so It can't be easily removed. If an employee is bringing in a product with a tag on it, it can be found and removed so it won't bark on the way out.
Theft can be a huge problem in a big facility that carries high cost small items though it's easy to segregate those items so they are handled in their own area. Any mixed order with some high value products would be assembled with the lower value stuff first and the unsealed box sent to the high value area for final loading and sealing. Just like people working in a bank vault, workers in a high value area should understand they are being watched much more closely and that's just part of the job.
When I had a manufacturing company and later when I managed another one, I ended the work shift 15-20 minutes ahead so things could be put away and cleaning done rather than tools being left out and work in progress left as whatever stage it was in when the clock said it was time to leave. I never saw that as man-hours lost. On the contrary, it was time saved the next day or at the start of the next shift not looking for tools or trying to figure out where work on something had been stopped. I also like having a clean shop every once in a while.