It's interesting how many discussions about encounters between models and full scale ac over club fields talk about 400'. Yeah, one reason is that's the measurement applied to models, but it's clear a whole lot of modelers aren't familiar with the specified altitudes for full scale.
Take a look at the FAR that lays out the law for general aviation:
Federal Aviation Regulations (FAR) Part 91prescribes the rules governing the operation of aircraft.
Sec. 91.119 - Minimum safe altitudes:General.
Except when necessaryfor takeoff or landing, no person may operate an aircraft belowthe following altitudes:
(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landingwithout undue hazard to persons or property on the surface.
(b) Over congested areas. Over any congested area of a city, town, orsettlement, or over any open air assembly of persons, an altitude of 1,000 feetabove the highest obstacle within a horizontal radius of 2,000 feet of theaircraft.
(c) Over other than congested areas. An altitude of 500 feet above the surface,except over open water or sparsely populated areas. In those cases, theaircraft may not be operated closer than 500 feet to any person, vessel,vehicle, or structure.
Your club field will probably have an "assembly of persons" on it, don't you think.
Of course it's always important to have a spotter watching for local traffic over your field, but truth is, it'd be a good idea to note any tail numbers you can read that appear under their required minimum altitude. And it'd be good to remember as well that any discussion about problems at your club field should be with an understanding that a full scale at 500 in YOUR gathering's airspace is definitely not innocent.