  We report the serendipitous detection with ALMA of the vibrationally-excited
pure-rotational CO transition $v=1, J=3-2$ towards five asymptotic giant branch
(AGB) stars, $o$ Cet, R Aqr, R Scl, W Aql, and $\pi^1$ Gru. The observed lines
are formed in the poorly-understood region located between the stellar surface
and the region where the wind starts, the so-called warm molecular layer. We
successfully reproduce the observed lines profiles using a simple model. We
constrain the extents, densities, and kinematics of the region where the lines
are produced. R Aqr and R Scl show inverse P-Cygni line profiles which indicate
infall of material onto the stars. The line profiles of $o$ Cet and R Scl show
variability. The serendipitous detection towards these five sources shows that
vibrationally-excited rotational lines can be observed towards a large number
of nearby AGB stars using ALMA. This opens a new possibility for the study of
the innermost regions of AGB circumstellar envelopes.
