Okay, time to treat it as if it were Mustard.
Refer to the "ylw/mstrd" columns for "min" and for "shock"
Do the normal shock level for a night or two - brushing the entire pool especially persistantly problematic areas - then bump it up for 24-48 hours to the MA level of shock. make sure you treat pool toys, floats etc while at this shock level. Is it possible its hiding in a light niche or ladder/wedding cake steps? inspect solar cover, if you have one... that can be a source....
Keep us posted, and I'm sorry this must be very frustrating...