Minecraft Version: 1.18.1 (Fabric)
Mod Version: v5.0.0

If the player is wearing armor with Mending on it, and it is repaired with experience while they have their inventory open, the (un)equip sound plays, even though no armor is being (un)equipped. I think this is because the ItemStack does not match when the damage is changed (this is checked here) so the sound effect is played.

Reproduced on a Singleplayer world and a Multiplayer server.

Also occurs if the armor is damaged when the player is hit and the inventory is open.