That's the door for the video tape recorder.
Referenced here, on an F-15 forum:
That location changed between F-15 versions. IIRC, our F-15C/D at Soesterberg was in the cockpit.
The sine wave sign indicates an access panel for electronics. Two examples:
sbap.be; electronic comp. access
sbap.be; electrical bay access
The handle shape indicates a nearby anchor point for hoisting the plane.
Here's a collection:
In your photo, the forward facing protruding part is a forward radar warning antenna.
10 Left AFT Fuselage
AFT outflow valve
Water drains, service doors, waste vents
Air demand pump exhausts (2)
The only non-explicit hint in the flight manual is in the exterior inspection section above; searching the manual for "air demand" yields only one other result:
Sec. 6.2 Page 1 [...] Air Conditioning Packs
[...] Pack output is ...
— Sakurai, Seiya, and Matthew D. Fevergeon. "Door assembly for laminar flow control system." U.S. Patent No. 8,245,976. 21 Aug. 2012. [Current Assignee: Boeing Co.]
Each door is actually one door inside another, so they can open as a scoop, or an outlet (opening fore or aft).
During takeoff the door scoops in air, so higher pressure is created in ...