Something akin to this happened here this week.
Setup: this school is within a mile of the Selfridge Air National Guard base, a sprawling point of departure for fighters, transports, bombers etc. Very high security 24/7.
Event: one morning a guy walks into a nearby school in camo fatigues, tactical jacket etc. but with an empty holster. He's a former student in the National Guard on his 2 week active duty stint, working midnight shift security at the base. In his off time he went back to his school to get a job reference letter he'd arranged for from his former counselor. No biggie, and Guardsmen in uniform are in the area all the time.
Night shift janitor leaving duty sees him, panics & tells day janitor who calls in the cavalry.
By the time half the city's cops, helicopters. & the SWAT truck arrive the guy's back home cracking a cold one. Schools are closed & put on lockdown, etc. etc.
Later the circumstances were figured out and a "never mind" issued