There is a thread in the Advance Scripting forum about the limitations of the AI. Apparently the AI troopers all suffer from severe cases of deafness and tunnel vision - if you walk up behind them they usually never know that you're there.
Try making a dead simple scenario - bad guy facing north, you facing north 100m to the south of him. Now run, sprint, crawl, duck, dodge and weave your way up to him. You should be able to get right next to him before he wakes up.
Fire your gun just once however and he'll be on the ground in a shot !
I think that they fixed up AI deafness / tunnel vision in Resistance, otherwise I'm hoping they give the AI a good seeing to in OFP2 !