I would use a flashlight and small mirror to light up the filler area and see if you can see the top of the cap and if you can see it you can try to use a small piece of a metal coat hanger or something similar to move the piece over where you can reach it with needle nose pliers.
If not the only way to remove it is to remove the valve cover like it has been mentioned before.
I wouldn't worry that much about it though.
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