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Symbiotic star surveys have traditionally relied almost exclusively on lowresolution optical spectroscopy. However, we can obtain a more reliableestimate of their total Galactic population by using all available signaturesof the symbiotic phenomenon. Here we report the discovery of a hard X-raysource, 4PBC J0642.9+5528, in the Swift hard X-ray all-sky survey, and identifyit with a poorly studied red giant, SU Lyn, using pointed Swift observationsand ground-based optical spectroscopy. The X-ray spectrum, the optical to UVspectrum, and the rapid UV variability of SU Lyn are all consistent with ourinterpretation that it is a symbiotic star containing an accreting white dwarf.The symbiotic nature of SU Lyn went unnoticed until now, because it does notexhibit emission lines strong enough to be obvious in low resolution spectra.We argue that symbiotic stars without shell-burning have weak emission lines,and that the current lists of symbiotic stars are biased in favor ofshell-burning systems. We conclude that the true population of symbiotic starshas been underestimated, potentially by a large factor.

SU Lyncis, a hard X-ray bright M giant: Clues point to a large hidden population of symbiotic stars