Overengineering a way to know if people are in my university's CS lab

Nice project but it could be illegal. Check your jurisdiction.

Source: have done similar hobby projects for fun, which turned out to be illegal.

this isn't facial recognition

???

Who's talking about that?

why would it be illegal?

Generally, you cannot just record people without their consent; but this also largely depends on the jurisdiction/situation.

Almost nowhere (or actually nowhere?) are you allowed to set up a surveillance device into a space that is not public and it is not owned by you.

Generally, in the United States, you can, in fact, just record people. Legally speaking, that is, which doesn't make it a polite or cool thing to do. Necessarily.

If you're on someone else's property, they can of course set any number of rules, and trespass those who break those rules. But even there, recording people, if against the rules, is still not a crime. The crime is trespass, if this journalist we're speaking of sticks around after being trespassed off the property.