I reported "suspicious activity" once. An unfamiliar Piper Arrow parked overnight after a 100+ mile trip at our 400m untended strip when the pilot could have used an 800m runway with fuel and services 4 miles away. That and a cockpit left with absolutely no junk in it, which is totally unheard of anywhere in GA !
The border chappies were very interested, but I never found out if anything came of it.
G