We've a rogue in the party with proficiency and magic items assisting their Sleight of Hand. With a +12 and always having advantage, they roll, on average, a 25. They have gotten to enjoy stealing items and then placing them on my character. I'm not the GM, I'm a wizard.
Last time they shoved an item into my backpack which, for the sake of this question, my character was wearing on her back. With a roll of 28 the GM didn't even ask me to roll perception because at a nat20 I would've gotten a 23.
Although I hate the ruling I will go with whatever is decided, so I'd like to know -- in game, what can my character do to prevent this in the future? If the GM rules that high enough roll for Sleight of Hand will get into anything, how can I get creative in preventing this?