I don't recall this happening when I played previously but have experienced this three times recently, on different worlds. My sim is at home in the evening, the doorbell rings, it is someone completely unknown. They come in and chat until my sim needs to do something (toilet, food, even go to bed) then go to the computer and play games or chat til around 3AM when they get up, change to outdoors clothes, say "I've got to go now", and leave. They are now on my friend list though I usually never see them again.

Not a problem, just quite curious.

Been a L O N G time since I played vanilla, but pretty sure at the start of a save series in an EA town, a friendly neighbor pops over to welcome you to the neighborhood. Not sure if that's what you have going on?