If I put myself in the shoes of someone truly malicious, I would request a backdoor in the chip implementation itself. Hardware is hard. Once you put it in the customer's hands they can't change much. Hidden opcodes or combinations/uses of legitimate opcodes that trigger undefined behaviors on the platform as a whole are the way to go. Firmware on the other hand can be patched or replaced.

It's probably an Easter egg or an undocumented workaround for something.