Thrips are not really a problem on Sans, and I doubt it is pest related. The only pest that really seems to get Sans is a type of mealiebug and you will have no doubt if you get them.....which you don't. Pull the plant out and check the roots and soil. I typically lose a few large leaves like that over winter. It looks like most of them may be part of one pup?? The inner plastic pot looks misshapen......which often happens with Sans as they push out new underground rhizomes. Could water now be running AROUND the root ball and going out the drainage (without really moistening the soil?). How are the roots (Do they look healthy and abundant?)? The plant probably needs a good repot anyway. I would suggest cutting off dead/brown leaves. Removing plastic pot. knocking off most of the old soil. Repotting into that clay pot with fresh/new appropriate soil (probably good to add some extra perlit to whatever mix you are using). Note these plants can take fairly frequent watering in spring and summer when temps are warm and the plant actively growing. Of course much less watering during cool periods and shorter days.