I have had a few GTR's and S14's that show high knock figures for no reason, one in particular was a SR20 S14, on the Kmon is was silent, ran a very safe map in it, yet the PFC D-jetro would show up to 50-60 on decel and even at idle would hover around 15-20, at wot it would often hit 95, I went over every squar millimeter of that engine bay, not a single thing loose or contributing. 6 months later I did an engine build for more power, same thing with knock, did not change.
Eventualy I replaced the knock sensor at a regular service and it dropped to about 3-4 at idle and 15 odd on decel with 50-60's at WOT (usualy see 25's to 30's on similar setups) I then replaced the complete knock harness from sensor to ECU, didnt make a difference. I even tried to switch out the PFC's with its sister car running identical setup and still the car im mention would always read close to double,
What I did notice though, if you run a hard core twin plate or tripple plate clutch it can be a very good contributer, SR20's knock sensors usualy go balistic right before/as they chow the teeth off 3rd gear as well. Not saying somthing is wrong with your driveline, just that it could be somthing a little futher down the line thats causing your odd readings.
Good luck, hope you find the problem.