"Ehhhh... I'm not convinced; maybe we should wait and see"
[pilots get confused about weird dots on radar]
"OMG THE ALIENS ARE HERE"
* balloons
* drones
* hallucinations due to stress & hypoxia
* instrumental glitches
* various kinds of accidents & debris
* experimental aircraft made by humans
* dunno, look, people make stuff up sometimes
* combinations of all of the above
- the chance of alien life existing somewhere in the Universe is extremely high
and
- the chance any has come to visit Earth is extremely low.
The main reason for both is the same: space is really, really big.