Was there a reflection off the glasses? Wasn't watching HD, but assumed that is what happened.
my initial assumption was in agreement with Pistolero. but, i also thought that might've happened, because there was a moment when walt lowered his head but his glasses were still sitting on top of his head still visible, and it kind of seemed like that was by design, like we were supposed to notice that... but it wasn't terribly obvious either, in fact it didn't seem significant at all at first, but i watched the episode again and it seemed more like we were supposed to take notice of his glasses like gus had just seen a reflection off of them. i'm still not convinced totally, my gut just said gus is just super cautious and tends to be prepared for anything, but i'm not sure.
i'll go check out that interview with giancarlo esposito on the AMC site now though.