Oh okay!
Common Myshuno neighborly visits in all game play:
Pump you up, work out
I'm bored wanna hang out
Can I eat, smells good
Can I use your shower, mine broke
Sounds interesting in there, can I come in
Intellectual chat
Angry at noise
They all knock, you can ignore, accept, decline
Ignore is best because a conversation can lead them to think it's okay to come in
If you give anyone a key they will..
Come over nearly everyday and..
Want to get ready at your apartment
Have game night
Hang out with friends
Basically they behave as if they live there
This is often as a set time (evening) and is timed much like the "welcome wagon" but WAY more often. It's harder to send them home. I don't recommend it and they don't knock as the ones mentioned above. They just come in.
People walking in that your Sims know but don't knock...
They had a conversation in the hall or in the street of that district
The district common areas and hallway are an extension of your apartment with the exception of the actual house in the spice district because it doesn't have an elevator
All areas with an elevator need a front door because elevators can't be locked, such is the case with the penthouses and perhaps was the problem.
To solve the penthouse visits place a wall that allows the postal service sim to drop off mail in the mail box and allows room for the trash chute. I believe it's 4x3 tiles as a "hall"
Place a front door and then set the door lock feature as desired.