"There are no undiscovered weapons caches because we haven't found them. Therefore they never existed... wait a minute.... yeah, that's right."
I recognize your imbedded point

and can only say that the critical error in that logic is that caches or even evidence of WMD cahces have not been seen
at all. Even interogations of detainers and POW's have turned up nada, zilch, zipp. That's in stark contrast to the many regular weapons caches that have been found and from interogations they know there are many mnore out there they have yet to get to.
Regarless. Small arms weapon caches I don't believe the UN or US were tracking before the war. At least I don't recall the US administration railing before Congress about mountains of RPG's & AK-47's nor Powell whipping out irrefutable "proof" on them in front of the UN body.