Sigh. There's always one that
must ignore common sense considerations. "Show me the law that says I can't drive with both eyes closed!"
Actually, that requirement exists in military regulations for the storage of classified information and equipment in homes (INFOSEC 2). Precisely because we don't want thieves to simply look at the nice portable fire-proof safe, think "I'll open that at home when I have time", pick it up and leave with it.
I'll freely grant you, there's no explicit requirement that a small container must be attached to the structure. The requirement is:
(i) rendered inoperable by means of a secure locking device and stored in a container, receptacle or room that is kept securely locked and that is constructed so that it cannot readily be broken open or into, or
(ii) stored in a vault, safe or room that has been specifically constructed or modified for the secure storage of restricted firearms and that is kept securely locked;
This said, without being a lawyer, I can just about guarantee you that a small (i-e portable) container for a handgun that is not secured against unauthorized removal will not be deemed to be "secure storage" by a judge. There's no point in placing the handgun in the most resistant case in existence
if you let the thief take it home so he can break it open at his leisure!
And of course, there's a matter of degree to this concept, like for the unattended storage of weapons in a vehicle: it's a risk management issue and the idea is that the weapons are not usually left in a vehicle for permanent storage.