I had this game when it first came out and one of the patches added an entire underground headquarters for the arabs. Osama is there. One of the weapons you get is a WMD. It's basically a rocket launcher that shoots biological rockets - same effect as the rotting cows head. I played through and didn't find it.

It's behind your house, down a sewer pipe - at the start of the week it's blocked by some foliage, but I think on Tuesday or Wednesday it opens up.

it's actually always open , you can just walk through the foliage.