........look at the photo. The "cap is on"........
As a rangefinder user, this has happened to me more than once and will probably do so again
I think since I first began shooting I've always held the cap with my teeth while I shoot. If it's a long amble I'll stuff it in a pocket, but often I see something, I grab the camera, cap in teeth, click, replace the cap, move on.
I've never lost a cap, even the notorious Diana Mini cap (though I did replace the Great Wall cap -- it'll fall off if the dog sneezes), but I have to admit I have spent some time looking for my Rollei 35S cap after shooting, as I can't see it below my nose.