is it possible it just fell off? the little rubber bits that hold them on get tired with time.
Hmmm... I am sure it was still there when I parked the bike but I guess anything is possible. Just the same I got a great tip on this thread for using zip ties. I really hate to modify a rare, original part but putting a small hole in the side panel and using a black zip tie to secure it beats the heck out of losing a side panel to theft and/or it falling off.
I inspected the grommets a few weeks ago when I replaced the battery in the old girl. The side panels seemed to be staying in place well at the time.
I was working on the left side of the bike last week when I opened up the left crankcase to replace a gasket. Could I have inadvertently loosened the left side panel while I was working on the bike?