Lost valve cover nut

[ubertino]

Working on the valve adjustment and I lost one of the nuts that hold on the valve cover, it fell from the front left corner into the lower engine compartment. I've used an led light on a flexible stick and searched everywhere in the engine that can unreasonably be seen, with no luck. It's not under the car, I checked.

I've been cranking the engine by hand, and not had any problems with the nut interfering with the fan, belts or much of anything else.

How much should I worry about this, and have the car taken to a mechanic to take apart to find the nut in question (not the one with the wrench)?