With some patience and practice, and an oil drum, you can mount the wire spool next to the guard tower (then bring the oil drump up to the spool to give you an additional lift) and hop over the electrified fence when you are outside in the We’ve Get Hostiles chapter, at which point, you are trapped. You can go behind the building and see incomplete models.
I actually had never played HL before and thought that was how I was supposed to complete the level, before I finally realized I could enter the bunker.