dropped the coolant plug relief cover, should I be worried?

I was going over the routine oil, coolant level before a long trip. Next to the coolant cap, there is a tiny vent to drain the coolant run over, there must have been a plug, as I was touching the coolant cap felt the cap and it dropped into the side of the engine, nothing on the ground.

I assume it was there to easily "pop off" in case of coolent over flow. Should I be worried or get it replaced?

thanks,

gychang