If you point an arbitrage opportunity out to someone, and they take it, then ‘they didn’t notice it’ seems reasonable.
However, figuring out what someone has and has not noticed does sound difficult—perhaps in an intractable fashion. Even if someone saw an opportunity long ago, perhaps they took a better one and forgot it, or lacked the necessary knowledge to figure out the opportunity.
If you point an arbitrage opportunity out to someone, and they take it, then ‘they didn’t notice it’ seems reasonable.
However, figuring out what someone has and has not noticed does sound difficult—perhaps in an intractable fashion. Even if someone saw an opportunity long ago, perhaps they took a better one and forgot it, or lacked the necessary knowledge to figure out the opportunity.
Or maybe the opportunity was only available for someone who could do (super)intelligent follow up to the initial opportunity.