I think that TSA Officers are surprised because the vast majority of people with Pre-Check are experienced travelers and they simply dump the contents of their pockets into a pocket in a carry-on well before the checkpoint.

Random searches for Pre-Check ought to be extremely few and far between (an Officer at IAD Pre-Check recently told me that they had not had a "random" in 3 weeks). If you are experiencing these, they aren't random, they are likely due to something in your pockets which is metallic but which you don't realize is.

Just do what most people do and you will probably live happily ever after without a "non-random" secondary.