  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 Io's supply to Jupiter's 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
Jupiter's extended ($\sim 1000$ R$_J$) Na exosphere and Jupiter's 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.
