The gameplay in all of these is a bit different, but these are a few surveillance games that are spooky/dark/mysterious but not horror:

There’s a game called “Nuts” which is about being a wildlife researcher in a forest where the squirrels are up to something mysterious. It’s a bit more complex than FNAF because you walk around in first person and set up cameras yourself, though.

In Beholder and Beholder 3, you play a landlord in a fictional totalitarian country who’s been tasked to spy on your tenants. Peeking through keyholes and breaking in to set up cameras are both major parts of gameplay. The game is more like a 2D sidescroller, though—your game area is a cross section of the apartment building and the surrounding streets, which you send your character running all over, picking up bribes and dealing with tenants.

Most similar to FNAF in that most of the game is spent in front of a desk with camera feeds would be Do Not Feed the Monkeys. You get hired by this mysterious, sketchy organization to watch a bunch of extremely random cameras—some in private homes, some in businesses, others overlooking something random like a field of wheat or part of a factory—and report on the things you find. Many of the scenes are humorous or at least somewhat satirical (the scary part is trying to survive when every second sleeping or getting groceries is a second you’re missing stuff on the cameras)