Maybe it was a process server. Maybe it wasn't; maybe it was someone pretending to be a process server, with fake papers. Maybe criminals casing the joint?
If this happened to me, I would be very creeped out, and I would go to the police. IMO, there is no legitimate excuse for anyone to go to your house at night and shine a light in the window.
I would consider this criminal trespassing, or maybe even stalking.
Edited to add: in some jurisdictions, with laws like castle doctrine and stand your ground, behavior like that would be hazardous to their health.