Extrasolar satellites are generally too small to be detected by nominal searches. By analogy to the most active body in the Solar System, Io, we describe how sodium (Na I) and potassium (K I) $textit{gas}$ could be a signature of the geological activity venting from an otherwise hidden exo-Io. Analyzing $sim$ a dozen close-in gas giants hosting robust alkaline detections, we show that an Io-sized satellite can be stable against orbital decay below a planetary tidal $mathcal{Q}_p lesssim 10^{11}$. This tidal energy is focused into the satellite driving a $sim 10^{5 pm 2}$ higher mass loss rate than Ios supply to Jupiters Na exosphere, based on simple atmospheric loss estimates. The remarkable consequence is that several exo-Io column densities are on average $textit{more than sufficient}$ to provide the $sim$ 10$^{10 pm 1}$ Na cm$^{-2}$ required by the equivalent width of exoplanet transmission spectra. Furthermore, the benchmark observations of both Jupiters extended ($sim 1000$ R$_J$) Na exosphere and Jupiters atmosphere in transmission spectroscopy yield similar Na column densities that are purely exogenic in nature. As a proof of concept, we fit the high-altitude Na at WASP 49-b with an ionization-limited cloud similar to the observed Na profile about Io. Moving forward, we strongly encourage time-dependent ingress and egress monitoring along with spectroscopic searches for other volcanic volatiles.
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