One time I left a note on a nice-ish old Trek that was locked up to a signpost in a friend's neighborhood. Friend told me the bike had been there for months. A few days later, the bike was gone. Too much of a coincidence. The owner probably figured, if someone's interested enough to leave a note, maybe it's worth something, and so they turned around and sold it. It didn't show up on CL, but I didn't check eBay and don't have bookface. So at least it didn't rot away on that post, or get cut loose and thrown on a heap in the property room.

In any event, nothing to lose by leaving a note, esp. if you can use some semi-anonymous Email address.