  Observations of helium in exoplanet atmospheres may reveal the presence of
large gaseous envelopes, and indicate ongoing atmospheric escape. Orell-Miquel
et al. (2022) used CARMENES to measure a tentative detection of helium for the
sub-Neptune GJ 1214b, with a peak excess absorption reaching over 2% in transit
depth at 10830 Angstroms. However, several non-detections of helium had
previously been reported for GJ 1214b. One explanation for the discrepancy was
contamination of the planetary signal by overlapping telluric absorption- and
emission lines. We used Keck/NIRSPEC to observe another transit of GJ 1214b at
10830 Angstroms, at a time of minimal contamination by telluric lines, and did
not observe planetary helium absorption. Accounting for correlated noise in our
measurement, we place an upper limit on the excess absorption size of 1.22%
(95% confidence). We find that the discrepancy between the CARMENES and NIRSPEC
observations is unlikely to be caused by using different instruments or stellar
activity. It is currently unclear whether the difference is due to correlated
noise in the observations, or variability in the planetary atmosphere.
