I don;t think folks are using the term "flash" correctly - flash it seems to ne is mor elike brandishing, normally illegal by statute in many states, or at least actionable as a common law assault, absent justification.
I believe the term "flash" would be an un-intentional peek of a firearm in its holster,
Definition of "flash" #32 at Dictionary.com: "to display suddenly and briefly: She flashed her ID card at the guard."
I think the term conotates a brief
display - not inadvertant shirt rising over holster while reaching up for a can of peas.
Alternatively: She flashed her badge at the check point to get professional courtesy. The gamgster flashed his gun to othger patrons while standing at the bar.
"Flash" is not a good term for us folks.