It could be the invisible to radar top of a bloody great CB
- to support Centaurus - ALWAYS keep the scanner moving down every now and then, because, as C says, there could be a 'bg' cb that is bubbling rapidly upwards towards your level, and with 'bg' cbs the upward air motion can continue for several thousand feet in clear air making a nasty bump.
I ALWAYS use max gain (737) to 'find' the wx, and assess its 'severity' in Auto. It enables you, for one thing, to make a smallish heading change at 50 miles instead of a panic '40 right' at 20 miles when it appears in 'Auto'.
I seem to recall someone called 'Archie Trammel'??? in the USA used to 'train' radar techniques. It is often (sadly) missed out on line training.
A few years ago I flew with a Captain one night who avoided the island of Jersey - 'wow - that's a big b**rd'. It seemed a shame to say anything.........
Edit to say it is Archie Trammel - just Google.