In the US only law enforcement officers are allowed to frisk you for personal belongings (without consent) once you've been placed under arrest(?). It's considered assault to so much as grab something out of someone else's hand, let alone hold them against their will, frisk them and/or reach into their pockets to retrieve what may or may not be personal belongings.
There was no evidence or probable cause to suggest there was company data on the thumb drive. If I were to hold up a thumb drive as I was being terminated and my boss instructed me to hand it over, I could very easily refuse and legally defend myself from attack.
On the other hand, Eric was essentially begging them to review his data model. It was pretty negligent for the HR reps to not at least ask "Well, do you have any company property and/or data on your person that you'd be willing to hand over?" during the exit interview. There's nothing wrong with trying to influence a terminated employee into voluntarily surrendering property/data that's in their possession, as long as they're free to leave at any time.
Being allowed to pack up your things and leave is even a stretch. There are plenty of places that clearly state that your desk area is company property of which you have no rights over. Please do not bring in personal belongings, etc. While it's not frowned upon to bring personal belongings to your workspace, there is no guarantee you'll get it back if terminated. Possession is 9/10ths and if your personal property is being stored inside company property, it's a catch-22.
A lot of places will boot you out of the building and send your shit home in the mail, so if anything, the plot hole lies in the cardboard boxes. On the flip-side, they were there to create a dramatic effect and it totally worked.