The flashback describes some of the events of the original Half-Life. In the game, Barney, the security guard, offers to buy you a beer (pretty much every time you bump into him).|
Posing Notes: In the flashback, I made panel 3 using G-Mod for Half-Life, but the panels 4 and 5 are from screenshots I found online. This was my laziness combined with the limited options of G-Mod for the original Half-Life.
In Panel 1, you can see a small file cabinet with a plug resting on it, and above it, a port for the plug. In the game, you encounter some blue force-field walls that you have to deactivate by pulling the plug. I envisioned this the equivalent of Mr. Henderson's office door. When the plug is out, his door is open, if he needs to get some work done, he plugs in the force-field. (I think the only one who noticed it was my friend Bob, who thought the plug was an alarm clock.)
Also in Panel 1, the framing is wide enough to establish that he has a pulse rifle on his desk for later use.