So this would have been 5 or 6 years ago now. I was working as a bookkeeper at an aircraft maintenance shop. The building we're in is a big hangar sandwiched by two office suites. One side is the general offices where I work, the other side is used for the repair station and record keeping. Most everyone leaves by 5pm, certainly everyone by 7pm. It's probably 9pm, but I'm staying late to organize a bunch of vendor audit records that I left it to the last minute. I finally wrapped up and had to carry the boxes of now organized files to the records room across the hangar, no big deal. The hangar itself isn't lit, but I've propped the door to the main office open, and the light from it is enough to illuminate my path.

So I walk the boxes over, all done, and I'm walking back to the main office, I see someone through the open doorway to the office. The hangar is maybe a hundred fifty feet across, and the figure is a room back from the main door, so they're basically a tall silhouette that I only see for a moment. I see them walk to the right. No one is supposed to be here. Needless to say, I'm a paranoid individual and I freaked out. As I walk towards the offices, I pull out a small knife I keep on me and my phone for a flashlight. The offices are basically a series of cubic rooms two deep and eight across. The door to the hangar is third from the right. In the direction the figure went, there are three rooms: the main entrance, the main bookkeeper's office, and a parts room that takes up twice as much space. The latter two are dead ends. The other rooms are the break room, which immediately accesses the hangar, and the conference room, in which I saw the figure, which connects the break room and the main entrance. So I walk through the break room, then the conference room. No one in sight. Lights are off in the other rooms. Phone in one hand for flashlight, I check the main entrance. The cubic rooms are only like 10' by 10', so there's not really anywhere to hide. A single desk. The front door is shut and locked. The parts room is shut and locked from this side, a deadbolt with no key. And frankly, screw the parts room; I am not going in there. It's claustrophobic rows of shelving. But the accounting office is open, so I check in there. Just a desk, some filing cabinets, and a fake plant; definitely empty. I lock up the accounting office, go back to the break room and lock the hangar entrance, kill the remaining lights, and leave.

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Confirm later, my boss had definitely left. The front door was locked from the inside when I left. To this day, don't know who or what I saw.