  The progenitor stars of several Type IIb supernovae (SNe) show indications
for extended hydrogen envelopes. These envelopes might be the outcome of
luminous energetic pre-explosion events, so-called precursor eruptions. We use
the Palomar Transient Factory (PTF) pre-explosion observations of a sample of
27 nearby Type IIb SNe to look for such precursors during the final years prior
to the SN explosion. No precursors are found when combining the observations in
15-day bins, and we calculate the absolute-magnitude-dependent upper limit on
the precursor rate. At the 90% confidence level, Type IIb SNe have on average
$<0.86$ precursors as bright as absolute $R$-band magnitude $-14$ in the final
3.5 years before the explosion and $<0.56$ events over the final year. In
contrast, precursors among SNe IIn have a $\gtrsim 5$ times higher rate. The
kinetic energy required to unbind a low-mass stellar envelope is comparable to
the radiated energy of a few-weeks-long precursor which would be detectable for
the closest SNe in our sample. Therefore, mass ejections, if they are common in
such SNe, are radiatively inefficient or have durations longer than months.
Indeed, when using 60-day bins a faint precursor candidate is detected prior to
SN 2012cs ($\sim2$% false-alarm probability). We also report the detection of
the progenitor of SN 2011dh which does not show detectable variability over the
final two years before the explosion. The suggested progenitor of SN 2012P is
still present, and hence is likely a compact star cluster, or an unrelated
object.
