I've never left a note inside a bike, but I have read of recommendations to do so in the interest of theft recovery. I also have read old stories of people hiding cash and jewelry inside handlebars and seat tubes, but I haven't found one of those bike yet. I've also read of people on tours hiding some backup cash on a bike in case their wallet got stolen while on tour. About the only things I have found inside handlebars are some old dried up mud wasp nests, a screwdriver and some wire and an air nozzle took care of those "finds".