This thief is unrestrained by those pesky laws of physics. Leave what they want in a timelocked diamond-hard safe underground in a room filled with lava past a pit of crocodiles and it will be gone in the morning. Should they choose to, they could steal your underwear (while you're wearing them!) without you noticing anything, or they could steal a jewel from in front of a dozen attentive guards without any of them seeing it go. When they're stealing a thing that can't be stolen in normal circumstances, like landmarks, geographic features, or abstract ideas or concepts, such as knowledge, ability, or souls, it's Monumental Theft.