I reckon, but wouldn't swear on it, that ammo used in and held in garrisons is not included in inspect forces pages, at least the monthly use isn't, I'm almost certain of that.
The only way to be sure is to go through each garrison and compare its ammo in stock with what it should be from the garrison blueprint. Then at least you would know if the 6K is going in supply units to top them up. If not then it is the monthly training ammo. That is most likely I would say but its a bit of a guess. I wouldn't have thought they disappear, very unlikely.
If you de-activate tanks or helis that aren't actually in units then I believe they use far less training ammo.