We present observations of redshifted CO(1-0) and CO(2-1) in a field containing an overdensity of Lyman break galaxies (LBGs) at z p 5.12. Our Australia Telescope Compact Array observations were centered between two spectroscopically confirmed z p 5.12 galaxies. We place upper limits on the molecular gas masses in these two galaxies of M(H2) < 1.7 × 1010 and <2.9 × 109 M⊙ (2 σ), comparable to their stellar masses. We detect an optically faint line emitter situated between the two LBGs which we identify as warm molecular gas at z = 5.1245 ± 0.0001. This source, detected in the CO(2-1) transition but undetected in CO(1-0), has an integrated line flux of 0.106 ± 0.012 Jy km s-1, yielding an inferred gas mass M(H2) = (1.9 ± 0.2) × 1010 M⊙. Molecular line emitters without detectable counterparts at optical and infrared wavelengths may be crucial tracers of structure and mass at high redshift.