Fluctuations of the local ion rate of Li+ ions emitted during in situ monitoring of the catalytic CO oxidation on the nanofacets of a Pt field emitter tip with a lithium field desorption-microscope (Li-FDM) are utilized to study the Li surface diffusion. The 'virtual probe-hole' approach, which is based on the digitization of the Li-FDM video images, is used. This allows to analyze simultaneously the local fluctuations of ions originating from arbitrarily user-defined probed surface regions of nanometer size. The anisotropy of the Li diffusion on the spatially inhomogeneous Pt surface is directly proved and a slowing effect of the coadsorbed CO on the Li diffusion is observed.