  Since the discovery of charge disproportionation in the FeO$_2$
square-lattice compound Sr$_3$Fe$_2$O$_7$ by M\"ossbauer spectroscopy more than
fifty years ago, the spatial ordering pattern of the disproportionated charges
has remained "hidden" to conventional diffraction probes, despite numerous
x-ray and neutron scattering studies. We have used neutron Larmor diffraction
and Fe K-edge resonant x-ray scattering to demonstrate checkerboard charge
order in the FeO$_2$ planes that vanishes at a sharp second-order phase
transition upon heating above 332 K. Stacking disorder of the checkerboard
pattern due to frustrated interlayer interactions broadens the corresponding
superstructure reflections and greatly reduces their amplitude, thus explaining
the difficulty to detect them by conventional probes. We discuss implications
of these findings for research on "hidden order" in other materials.
