See the previous Preview Image for details on the Sentry Manual.
This scene is supposed to be Blue Engineer's Shed. The actual location is one of the sheds at Point A in the cp_gravelpit map of Team Fortress 2. The point of all the clutter is to provide a number of easter eggs for those with keen eyes (THE DOPEFISH LIVES!) as well as implying that the TF2 guys somehow have been able to travel to other Valve game universes and the Engie has swiped quite a collection of souveniers for himself.
Unfortunately, a side-effect of my quest to clutter up the shed with as many interesting objects as possible, as well as a wide variety of interesting characters (for reasons you will see in the comic) as possible, as well as throwing in any map that looked even a little interesting into the maps directory, all led to Garry's Mod having a anueyrism and I had to wipe and reinstall everything. This meant losing all existing scenes at the time.
This time around, I was very careful with what I installed, and I'm pretty happy with my options.