  We explore the H-alpha emission in the massive quiescent galaxies observed by
the KMOS-3D survey at 0.7 < z < 2.7. The H-alpha line is robustly detected in
20 out of 120 UVJ-selected quiescent galaxies, and we classify the emission
mechanism using the H-alpha line width and the [NII]/H-alpha line ratio. We
find that AGN are likely to be responsible for the line emission in more than
half of the cases. We also find robust evidence for star formation activity in
nine quiescent galaxies, which we explore in detail. The H-alpha kinematics
reveal rotating disks in five of the nine galaxies. The dust-corrected H-alpha
star formation rates are low (0.2 - 7 Msun/yr), and place these systems
significantly below the main sequence. The 24micron-based infrared
luminosities, instead, overestimate the star formation rates. These galaxies
present a lower gas-phase metallicity compared to star-forming objects with
similar stellar mass, and many of them have close companions. We therefore
conclude that the low-level star formation activity in these nine quiescent
galaxies is likely to be fueled by inflowing gas or minor mergers, and could be
a sign of rejuvenation events.
