The TSA notified me when they opened my bag a few years ago, with a polite leaflet placed in the bag. I would not have noticed otherwise - the contents appeared undisturbed. I don't know if this is standard or current practice.
I suspect it probably hasn't changed much.
The only odd thing I can think of is I have a tiny pair of pliers in one pocket due to an iffy zipper. I also typically put a laptop in there instead of carrying it. Maybe that combo sets something off.
It's at least pretty good proof that these checks aren't random (which is truly a good thing).