Are you sure you're not thinking of "SmartDay" days that are part of the SmartRate program?
Flex Alerts are CAISO and ultimately about grid stability. SmartRate/SmartDay are ultimately about marginal cost of production on PG&E. The two are certainly correlated -- at the very least, a Flex Alert day is almost guaranteed to be a SmartDay.
Notably, the SmartRate program is capped at 15 days per year, and in practice PG&E will keep a few in reserve for surprise late season events, but even if there are no Flex Alert days they're still going to be called on electricity-is-expensive-even-if-the-grid-is-stable days.
