I found one in the giant room right before the boss Mistral. There is a tunnel where you find a bunch of cardboard boxes piled up in a dead end and if you start chopping away at them, it'll reveal an enemy hiding in them. You'll then get a codec message saying "I wonder where he got that from", obviously referencing Snakes iconic method of stealth.
Yeah, they're the MIB (Men In Boxes). I already got the achievement for finding them all. If you look at the "collection" tab from the main menu after killing these guys, you get to see their cards, showing them in dramatic poses with their trusty boxes.
the sombrero is the same as the ones in the movie the three amigos, though the poncho is closer to the man without a name in the clint eastwood movies. Not to mention the whole "detroit pioneered the use of cyborg policemen..." comment. This game is Full of easter eggs.