Usually it's a 'helpful' note I leave ie 'noticed your driver side rear brake light not working' or 'noticed your passenger side rear tyre is flat'.
If someone's parked up my @&):, leave a note telling them what I think of their parking skill. This is really pertinent when someone 'overflows' into MY car parking bay; gives me the screamin' 'heebie-jeebies'
I was in a car park once near XMAS, & the car next to mine had 'antlers' coming out of the front doors! I just cracked up, it looked SO cute & funny.
So I left a note saying 'thank you for giving me a really good laugh, & they look good. Merry Xmas'