That's probably your coolant cap, it on the left side of the bike. If you have long skinny fingers after taking the side cover off and also the seat disconnect the fuel hose from the tank, then lift the back of the tank up you might be able to get the cap back on.

 

As @DirtyDaveCR said Definitely the coolant cap. Mine fell off when I replaced air filter. I lifted tank up and was able to reach it. Left side.

 

Thanks, it is the 'Pressure Cap Cover #5416165. The number was on the inside of the cover. I have to lift the tank? I was afraid of that. I just filled up.
Thanks
View attachment 878737

 

Maybe just a little lift. I don’t think you have to take all the way off

 

To be clear, it’s the radiator cap cover. It’s to protect the tank from the metal radiator cap from what I can tell. It falls off pretty easy when you’re putting the tank back on.

 

I just had to lift the tank, much easier the second time around.

 

Glad to hear it was easy!!!