Your pic is really small but is that an almost perfect circle? That would be REALLY odd to be simple rust. Wonder if someone could have patched it before and used a circular piece of metal to patch it and it's now rusting around the edges of that? The size looks almost like the size of a cover to an electrical junction box or a coffee can. Have you looked thoroughly on the backside?
SouthernGypsy's current Jeeps:
1983 Jeep CJ-7 Limited 4.2L ~ Daily Driver, Prepping for a 2 month Tour of America Summer 2014.
More Mopar Muscle Cars, Grand Cherokees, Cherokees, and ATV's than I care to count!