Yeah, I believe a skill check frequency reduction is coded into these perks. I once used Scavenger for a tome challenge (I think it was empty a recharged toolbox or so). Brought a brown toolbox without add-ons, used it on first gen, and after using it up there was not a single skill check for the remainder of the gen. It took me four matches to get enough skill checks to recharge the toolbox and as I got my recharge the last gen was just done (I probably could have depleted it by saboing hooks but I didn't think of that). In the sixth game I finally got the recharge in time (and when I wanted to use it up I noticed a red repair bar ... looks like a perk that works only once every couple of matches needs a built-in counter to itself, no idea how such garbage made it beyond the rough idea stage.)