The cops were there on a check welfare, which means they have to check every room to see if someone is there that needs help, but is unable to respond, sometimes sick or dead.
I have found several dead, from various causes.
Now as to why they were there on a check welfare, is an entirely different issue, but the courts have recognized the validity of an officers presence due to a check welfare.
Plain view simply means it was out in the open and they didn't have to open drawers or search for what they found.
The legal basis for their presence, the finding of the weapon and the warrant that followed are well founded in the legal system.
All this Weapon of Mass Destruction is the biggest load of crap I have ever heard.
If I had seen the same SBS, I would have said, "cool" and left it at that, since I find the law to be ridicilous.
Here is an explination of the plain view doctrine. The actions of the officers appear to have met the criteria of the plain view doctrine.
Plain View Doctrine