only thing I have spotted whilst flying was back when I was doing my night VFR rating.
Was out on a navex one night, moonless, so very dark.
During the cruise I was doing a scan out the windows when I saw a massive fluro green glowing object descending and moving past us like we were standing still.
My guess, it was a decent size meteor or meteorite.
Scared me s**tless though in an otherwise uneventful and boring cruise!
Quite a sobering thought when you consider if you were extremely unlucky, it could actually hit you mid flight.
Only other instance where I have seriously questioned what I saw, was one night when I was living out near Wellington in South Australia.
I woke up at about 2 or 3 am due to a thunderstorm passing overhead.
Plenty of thunder and lightning with very heavy rain.
While I was taking a quick squiz at the storm out the window, I spotted a steady white light, sort of looked like a ball shape, doing a very wide orbit over the property I was at. My guess would have been 1,500 to 2000 feet in altitude. It was moving quite fast. But below the cloud base.
I observed it make 2 big orbits over the property, then it just dissapeared.
No idea what that was, but I am certain it wasnt an aircraft.
No one in their right mind would be flying at that time of night, under cB, at low altitude and at a spot that has no significant aviation relevance (IFR reporting / holding point etc).
Ball lightning?