There's a huge difference between a company with its own building, and a company that shares a building in some way with other companies.

Many I've seen have it setup so that if you get past the security guard at the lobby, you effectively had full reign of the entire building, including many companies that wouldn't lock the doors or common areas.

~400 person company spread across a few floors, but only one kitchen. It wasn't weird for people you didn't recognize to come off the elevator and get snacks to take back to their floor.

I worked somewhere with a few hundred employees across 3 floors. If someone wearing business casual walked onto our floor I would have no idea if they worked for us or not.

I work at a company of ~200 people and I already don't recognize everyone. Seeing an unknown face, I just assume they are from some distant team that I never had to interact with, say hi and move on.

We have nearly a 1000 people in my building. I don't track every rando that walks by, nor reasonably could I.

Another aspect besides not recognizing everyone from your company is like this- even if someone knows for sure that a person from a different company is helping themselves to snacks, people are may avoid pointing it out. People may prefer to avoid conflicts or make someone else look bad. They are more likely to act if they see someone stealing from their desk, home, etc. That's kind of their domain.

Also, a few other things may also be there- people won't make noise if someone steals snack packets, but they may make noise if someone steals laptops.

Also, if one person steals it may get pointed out more than if a lot of people steal- where stealing is culture, etc.

Feels like every time I drop by the office there's 2-3 new faces I've never seen before.

People I know seems to not take issue with them being there, so I'm sure it's probably fine. Fine enough for it not to be my issue to deal with in any case.