We explore the { H \alpha } emission in the massive quiescent galaxies observed by the KMOS ^ { \mathrm { 3 D } } 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 [ \ion N2 ] / { 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 M _ { \odot } /yr ) , and place these systems significantly below the main sequence .
The 24 \mu m-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 .