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Possible, but let's assume they knew about it before AI could identify it without too much hand-holding... 6-12 months or so ago give or take?
That means they had time to generate keys in the entropy space and check addr indexes to get an idea of how much they could steal... which has been far more than anyone other than a nation state can reasonably wash.
If it was an inside job, thats 5 years of betting no one notices.
It seems like all the rationale would point to doing this some time ago if it wasn't recently discovered, and that makes the AI ad-hoc discovery by a random in the last few weeks or months the most plausible imo.
AI could have caught them off guard?
Lots of people are saying that this would be very easy to miss in review, so it seems plausible that someone who became aware of it would also come to that conclusion.