It's very toxic and corrosive, so it should only be used in very low concentrations. It will appear in soaps as that's what turns the liquid into a hard block. The best soap bars shouldn't have any lye as they will evaporate completely. But some factories will not be too strict, and some lye will get inside the bar. Make sure that any products that use lye use a high percentage of lye (more than usually required), as this will help dissolve the lye.