A thought occurred to me. Did Bnei Yisroel know that Makas Choshech was going on, given that they could see everything normally?
Presumably in the stronger phase of the makoh they would have noticed the Mitzriim not moving, but they could have thought that הא גופא was the makoh.
If this is the case, possibly their going around the houses of the Mitzriim to check out the loot shows a degree of mesirus nefesh, if they were unaware that the Mitzriim couldn't see them.
Thoughts?