Find the person responsible for the place.
At a terminal, station, aircraft, or crowded event, alert the nearest employee, crew member, security officer, or police officer. Describe where you are and what you observed. In an immediate threat, move away and call 911 when safe. Do not leave the concern only in a social post.
Keep the report specific.
Say the gate or platform, time, exact words of a threat if heard, the action that concerned you, and whether it is continuing. An unattended bag by itself has many ordinary explanations; do not touch it or decide what it contains. Tell staff its location and any other facts you actually observed, then follow their instructions.
Do not interfere with the response.
A traveler cannot see the whole security picture. Avoid following a person, opening a bag, blocking an exit, or filming in the middle of an active response. If officers ask you to move, do so. Make a short note afterward while details are fresh if staff need a follow-up account.
Further reading: TSA security screening and reporting guidance.