Looks like these guys are using technology beyond their ability. I could clearly see the third guy right before they took out the first. They also seemed to take their sweet time identifying the targets; the infantry could probably have either hidden and gone un-noticed or ran out of the field of vision. They just got lucky guy #3 crawled out there and they saw him.
Also, I'm not sure about how well it would work at that proximity but some kind of heat sensor might have been useful if they could throw that screen up.
Ok, well, that's just a layman's view asserting judgement on what I can tell. Any experts want to shed some light on that?