Well the only reason I think sligs couldn't see you in the dark with their night vision goggles is because
A. They are probabally not night vision. or
B. Since sligs have room tempeture IQ'S they wouldn't notice Abe sneaking in and out of shadows because Abe is being really stelthy and quite smart enough to get past a stupid slig. In the Stock Yards its at night and the sligs can't see Abe in those shadows but when he steps out of them in the moon light bam. My guess they are night vision visors but their view really sucks.
You ever put on a helmet with a visor similar to that of a slig? The view really sucks. I know this because my friend has a bunch of Star Wars helmets and let me tell ya the view from inside the helmet of a Storm Trooper really and absolutly sucks. I could hardly see a thing but at least I could see sort of.
Maybe the slig's Eye ware is similar to that where they can hardly see out of their visors and goggles which is my guess and can explan why they can't see Abe with night vision some times.
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