Don't you have to wear/hold them to get the malus ?
Maybe it's normal to be unable to see sneaker chars, Shangy... 
Well, like I said, it seems thats sometimes we have few David Coperfield Clones on the battlefield, and it's kinda...annoying.
Yes, it's normal to be unable to see sneakers, that's why I don't want to see a feature such as "when you take a loot, you lose the sneak". Ridiculous.
And, of course, when you take heavy loot, you got the malus, based on the weight. You don't need to wear it to have this. And furthermore, you cannot wear any armor best than leather or you will face the consequences of your useless sneak.