I presume you are talking about your cossie, in which case could it have been a nut that fell down rather than a bolt

Either way i would get it out mate because the last thing you want is a bit of metal, of any size flying around by the valves when they are going up & down. It could cause some serious damage to the springs, valves and/or the head itself

I agree with FletchCossie go get a telescopic magnet and fish it out