I found a rubber cap under my seat in my 2015 honda accord where does it go?

It is about the size of a quarter and has the honda logo but i can't figure out where it fell off from

Probably one of the power connectors (called at one time cigarette lighters) which are now fitted with caps to protect them when empty. If you have ever accidentally left one open, and dropped a dime in it, you will know why they are needed.

A rubber under the seat? Look around

I would ask the dealer.