chetchat, you are getting into major gray area when you talk about storage of a firearm. We've been leaving Class III just as you described at a Class III dealer when going on long vacation or business. Have had a number of guns (Class III) stolen in the area when owners are out vacationing. We have talked to ATF about what we do and have never been able to give a definitive answer as to whether it is OK or not. They don't even know, but kind of look the other way because we're trying to do the right thing.
AND, we also have corporate NFA weapons holders being visited by the ATF. No warrant, reason or probable cause. Again, corporations have no rights or at least ATF seems to think so. Anyone contemplating the purchase of a NFA weapon via a corporation (or forming a corporation for the sole purpose of owning a Class III firearm) should go and talk to a lawyer first.
From Miami! My old barrio.