> Former Yahoo employees told Reuters that security staff disabled the scan program after they discovered it, and that it had not been reinstalled before Alex Stamos, the company's former top security officer, left the company for Facebook last year.

Quite a fitting location for surveillance advocate.

I'd like to know what went through the minds of the security staff who stumbled onto that find, and how the heck did it stay running for so long?

Nothing goes a long time without breaking in the ops world. I don't believe that they just happened to stumble onto it.