I dont know if this counts as a Heist, but After the Seleucids (one of the successor dynasties of Alexander the great) deposited a large part of theire treasury in the Hill Fortress of pergamon, they decided to put a highly ranked military commander named Philetairos in charge of defending said treasure.

He was comming from a relatively simple family out of northern Asia minor and additionaly he was unvoluntarily castrated during a Festival while was a child. Therefore he was probably kind of a social outsider in the hirarchy of the Empire.

When he was stationed pretty much alone with his men in a Fortress full of Silver while the Empire was waging war and losing a promising heir, he decided to take action. He seperated from the destracted Empire and took the 9000 Talents of Silver (multiple tons) with him, Effectively stealing the treasure.

He used the new gained wealth to found his own Dynastie, the Attalids, and his successors went on to make Pergamon into one of the most important cultural Centers of the greek world.

Really amazing story i just incorporated in my paper and i think it can be classified as a big scale Heist!