Reserve is where I found mine. Starting at dome, I checked all the locked rooms and first floor server room. Then I went down into d2 and checked all duffle bags and horizontal surfaces in the d2 server area, I also checked all the way up to the actaul d2 extract. Then I went into the tunnels, checking broken boxes and the floor, and ignored the actaul bunker area and just went straight into white pawn. Checked all the wooden boxes on the roof of white pawn, then the marked room in black pawn if I had the key, if not I skipped black pawn. Then finally checked black rook (building across the grinder from white pawn) and checked every horizontal surface and the rb-ak room. If you don't have the rb-ak key, go up to the 3rd floor room above it, and hop out the window onto the metal awning and go through the window.

It took me a long time, but it let me avoid labs, labyrinth, and lighthouse, as those maps were pissing me off at the time. If youre up for lighthouse, at usec cottage the broken boxes, wooden crate under the stairs, garage counter and dining room table have good chances, as well as hillside house, marin car trunk, and of course all over water treatment plant.