Ya but if someone did find the room behind the painting chances are they are going to break down or around the door or place a button of their own next to it to get it. We need some sort of protection hiding it is all well and good. Sneaking doesn't make you disappear from a map either so it's only a matter of time before someone finds the stuff.
I thought I read somewhere that 4J was considering or looking into some way to stop people from greifing in certain ways. The funny thing is that there are no dedicated servers and there are options to close your game so that is at least a layer of protection. I guess everyone has those friends that still think its funny to mess with you so it's probably worth implementing.
Maybe if 4JSteve or 4JStu see's this they can confirm or not whether that particular rumor is true or not since I'm not sure.