Unlike other striker-fired guns, the P320 is "effectively fully cocked at rest", since its striker is under constant spring pressure, which is released when the trigger is pulled. Most such guns, including the P320's military variant, have external safeties (like thumb safeties), which the P320's civilian variant lacks. According to gunsmith James Tertin, this is a rare and "uniquely dangerous" configuration
(from https://en.wikipedia.org/wiki/SIG_Sauer_P320).
MCO 5500.6H (Arming of Law Enforcement and Security Personnel) requires Condition 1, which is magazine inserted, round in chamber, slide forward, safety on ... After reviewing the security camera video footage, the mishap investigator concluded that P1 did not mishandle the weapon at anytime while on duty at Gate 1 prior to the weapon discharging. From the evidence and statements from the persons involved, it is apparent that the weapon fired while on safe and secured in the holster.
(from https://npr.brightspotcdn.com/86/f5/d87374f8497a81db30e2463a..., linked to by the article)