It really could be almost any case contact. One thing to check (assuming that the standoffs aren't a problem) is the backplate / ports cover.
There's also a small chance that the power button / reset button are grounded somehow.
Yeah I actually removed the motherboard, re did the standoffs, checked the back plate and even bent the tabs so they were tighter against the motherboard port clusters.
I've got 4 more N200's coming in today so I may just have to build in one of those and see if it resolves. The guys from Cooler Master said that sometimes its the I/O cluster and so they recommend RMAing it. Screw that, going to send it back to the Zon for a replacement.