Definition
An unmanned aircraft system whose presence, behavior, or operation raises reasonable concern that it may pose a threat to safety, security, or critical infrastructure, or may be operating in violation of regulations. Suspicious activity can include flight near sensitive locations, unusual flight patterns, operation in restricted airspace, or any behavior that suggests intent to surveil, interfere with, or endanger people, aircraft, or facilities.
Plain English
A drone that is acting in a way that doesn't look right -- flying where it shouldn't, behaving strangely, or near a place it has no business being.
Context Anchor
Pilots may encounter this term in airport security reports, air traffic control notices, or pilot reports about unmanned aircraft seen near airports, aircraft, runways, or other sensitive areas.
Derivation
Suspicious' comes from the Latin suspicere, meaning 'to look up at' or 'mistrust.' UAS stands for Unmanned Aircraft System -- the drone itself plus its control link and operator. Combined, the term flags a drone whose activity invites mistrust.
Why Pilots Care
A suspicious UAS can create an immediate collision risk and requires prompt reporting so authorities can investigate and clear the airspace.
Intuition Check
Suspicious does not mean you know the operator has bad intent. It means the unmanned aircraft is being seen in a place or manner that gives a reasonable cause for concern.
Example Sentence 1
ATC advised all aircraft on approach of a suspicious UAS reported hovering near the approach end of Runway 27.
Example Sentence 2
ATC asked all arriving aircraft to remain vigilant after receiving a report of a suspicious UAS in the vicinity.