With regard to the notes

It isn't
their content

the riotous spelling,
the cramped, jagged hand

it's not
even the use
of invisible ink.

It's the fact that they've stopped.

Birkenhead-born Ken Cumberlidge has been writing poetry, songs and stories on and off for 40+ years.  His most recent work has appeared variously online (Algebra of Owls / Allegro / Ink, Sweat & Tears / The Open Mouse / Picaroon / Spilling Cocoa… / Strange Poetry / Snakeskin).  Since 2011 he's been based in Norwich, where he can be seen muttering and gesticulating in the company of an embarrassed-looking dog.  Don't worry - the dog's fine.