Flying Thingy

A few days ago my security camera was triggered during the early hours of the morning by something I can't explain.
The security camera has a 'line crossing' feature which is enabled on 3 sides of the fence in my garden, so that any human crossing the line (climbing over the fence) triggers an alert.
Never before has anything flying, such as a bird or butterfly triggered the alert and in fact, squirrels and cats often walk along the fence without triggering an alert.
Could the flying thing be a moth? or could it be something more sinister?