But you may still get opportunistic servants who just grab an item and sell it to some criminal contacts with a description where it's from. These sell it on again, until it gets to the boss of the criminal organization who knows just the person to sell it to. In the real world, there is an idea that all people are six or fewer social connections away from each other (link to
wikipedia).
6 Months means an item can change hands dozens of times, and find its way to a high level spellcaster with bad intentions quite easily.
Some mages want to rule the world, and need orders of magnitude more cash than you normally get in a campaign? Out-of-character, it's usually not that the players don't know what to spend money on. They just don't get the opportunity to spend it in the campaign.
Also, bards and sorcs can also cast Teleport, and may be more chaotic in alignment, and steal some cash just for fun.
If you have a treasury with many millions of gold, sure you keep a few golems. But a golem costs in the 50k-100k range, so if your entire fortune is in that same range you will not have such powerful guards inside the treasury to guard only against teleporting baddies.