Curiously the issue I have had is not with the pads in my underpants, but the spare in my back pocket. I had left it there assuming the scanner was only detecting metal, but after being stopped a couple of times for a body check now I move it into a pocket of my jacket (along with keys, wallet, phone) which goes in a tray.
Now rarely stopped, and when I am it is probably one of the random checks that everyone gets caught by.
To be fair, once they found the "forbidden" article and I explained that it was something I routinely carried following prostate cancer surgery they accepted the situation. And the probing was not too intrusive, it used a hand-held scanner and was the same as others have had on random checks.