A free mod for Witcher 3, by menschfeind13.

Invisible Weapons and Swords on Roach is a mod that lets you view the swords and weapons that you don’t have in your inventory when you’re playing as Geralt. When you’re not holding any weapon, it’s shown on Roach, and if you draw a sword or a crossbow, it disappears from Roach. The only way to make it work is to buy a new item (Medallion of Concealment) from an ofiery merchant. Alternatively, you can use a console command

ghost_of_the_sun

which will give you the new item.

If you want to disable the mod functions, you should buy your weapons and exit your inventory, put that item in your stash or drop it somewhere, and then re-equip your weapons.