So I'm currently buliding my lab (very, very, very, very expensive.) and I'm bringing it online section by section as I can manage it. Main floor, admin, plant biology, medical technology, chemistry, robotics and so on. Anyway I just finish the construction on the new elemental science wing (sub atomic particles, elements that make up life, nanotubes, etc all the stuff that are too small to see) and I'm sitting there doing my usual convert everything to energy efficient devices. Anyway I'm just sitting there watching my sim work as a random person walks in, sits down at the areas reception desk and begins working on the computer there. No idea who she is, no attempt to interact with my sim just strolls in and assumes the secretary seat to work. This by the way is in a place where to get to said seat she had to come in, walk past ALL the ground floor computers, past all the admin basement 1 computers and use the one here two levels below ground.

I knew that people were getting lost in this lab but this is getting creepy I'm sending my sim to do something and finding people just taking on the lab roles in a residentail building. The gardener and maid sure I hired them but the guy using the recycler (no idea who he is), this random woman in the secretary seat (never met her) that other apparently pregnant woman working on the solar panels on the roof again NO IDEA WHO SHE IS OR WHY SHE'S HERE. I have 6 people wandering around my lab one is my sim, 2 are employees the other 3 just showed up and started working on stuff here.

If you want it to stop, you can set the main door/gate to "no trespassing" and select who you mean. You could also use a lot trait to turn it into a private lot. On the other hand, it lends credence to the lie I tell my kids when they don't want to leave the store...

You say creepy, I say annoying.
there are a couple of code lines to prevent sims with certain roles from trespassing onto the active lot, and getting in uninvited
it exists in the code of many roles (like the City Living weirdos, for example), but ever since Island Living EA just forgot that they have this technology..
or they might think it's funny when random sims make themselves feel at home where they shouldn't, I don't know.

If you've got Eco Lifestyle you'll notice it more frequently
sims just trespass onto the active lot, get into your sim's home lot and do whatever they want.

now instead of adding those restricting code lines in the tuning files of those roles
EA have made a completely different system in which you need to click on your sim's front door to disallow certain sims from stopping by
if it was working it could have been fine, but I've seen someone try this out in a lets play and seems like it gets reset after you leave the lot and come back to it..

if it fits your needs with that lot, you're allowed to enjoy that if you will
but I find it annoying and I just fix for myself as I go.

I have the front door on my home locked to keep them out but this is meant to be a lab and annoyingly the two big, heavy security doors don't actually lock. One hasn't got the ability at all while the other can only be opened or closed from one side meaning the maid/gardener wouldn't be able to access it. I may need to to just swap for a normal door till I've finished the lab as I'm sort of living in there while I make the money to build it. I just have to make sure all random sims have been actually chased out of the lab first so I don't lock them inside hmmm I wonder if I could CAS cheat and turn them into an animal . . .

Thanks for the replies.