I don't think this is a matter of screener discretion. I think they play it both ways: if a screener wants to deny something reasonable (3.5 ounce container), he/she hides behind "I just have to follow the rules". That same screener may very well cut corners if he/she is lazy - those 'shortcuts' are 'discretionary'.

At least sometimes, I think it's a matter of experience. An experienced and attentive x-ray screener probably feels confident that if he/she is 95% sure what an object is, if the pax confirms it, the item is good to go without a bag check.

Another TSO here who subsequently deleted all her posts said they had an old guy on the x-ray who called 'bag check' on almost every single bag. They called him the 'bag nazi' - he saw illicit liquids in almost every bag and called for a search (nothing there). She speculated offline that it might have been because he was old/drowsy/poor eyesight and was afraid he might miss something or that it was because he didn't like the younger TSOs around him chatting while they stacked tubs/monitored the belts/etc so he kept calling for bag checks to break up the party.

I honestly wonder if some of the gropes aren't the result of inattention in the booth - the voyeur in the booth is fooling around and not paying attention, so he/she calls for a grope on the pax he/she misses.