I’m really late, but I lost Voltix’s yellow tentacle thing somewhere in my house. Luckily I have two other Voltix sets that do have the tentacle. I also lost all of Jawblade except for an armour shell, his weapons and the instructions. Most of the ammo for my Glatorian and Breakout sets went into the unknown during ‘exciting battles’.
And lastly, while this isn’t Bionicle, all the rocket arm Super Battle Droids from my AAT went missing apart from a single torso.