Magic Barrier
It was in your sights. Sitting neatly on a shelf surrounded by other stolen treasures. A shimmering blue field surrounded it, but despite being unable to use magic, you know exactly how to dispel it. At least, kind of.
Magic was powerful but everything had its weakness, and in this case the weakness was a mirror. With enough force and a little luck it would reflect the magic back into itself, leaving just enough time to grab your artifact and get out. You weren’t exactly sure how it worked but you knew it would.
With a steady hand you flick the pocket mirror through the air, landing it perfectly on the shelf. In a single motion you snatched your artifact and threw it into the bag at your side. Just like you’d practiced. Now all that was left was escaping.
You turned around to sneak out the way you came in and your heart dropped. Another barrier on the door. Of course the door was trapped, how could you have been so stupid?! Even if you could get your mirror back, it certainly wouldn’t make a big enough space for you to squeeze through. You weren’t sure if anyone had been alerted, but surely someone would come in here eventually and see that the trap on the door had been tripped.
The only things you could think to do were risk climbing out the window or hide and wait for someone to return..