the locked chest was an april fools joke, it only remains in the list of possible blocks because nothing else has replaced it (like the nethrack half slabs replaced the smooth slab)
After april first all of the "joke blocks" where turned into a rather pretty but glitched appearance, it also happens to dissolve almost as soon as it exists. If you don't like seeing it in NEI, go into the dropdowns and remove it from the list of ones that display.