No random transits in CHEOPS observations of HD 139139

HD 139139 (a.k.a. 'The Random Transiter') is a star that exhibited enigmatic
transit-like features with no apparent periodicity in K2 data. The shallow
depth of the events ($\sim$200 ppm -- equivalent to transiting objects with
radii of $\sim$1.5 R$_\oplus$ in front of a Sun-like star), and their
non-periodicity, constitutes a challenge for the photometric follow-up of this
star. The goal of this study is to confirm with independent measurements the
presence of shallow, non-periodic transit-like features on this object. We
performed observations with CHEOPS, for a total accumulated time of 12.75 d,
distributed in visits of roughly 20 h in two observing campaigns in years 2021
and 2022. The precision of the data is sufficient to detect 150 ppm features
with durations longer than 1.5 h. We use the duration and times of the events
seen in the K2 curve to estimate how many should have been detected in our
campaigns, under the assumption that their behaviour during the CHEOPS
observations would be the same as in the K2 data of 2017. We do not detect
events with depths larger than 150 ppm in our data set. If the frequency,
depth, and duration of the events were the same as in the K2 campaign, we
estimate the probability of having missed all events due to our limited
observing window would be 4.8 %. We suggest three different scenarios to
explain our results: 1) Our observing window was not long enough, and the
events were missed with the estimated 4.8 % probability. 2) The events recorded
in the K2 observations were time critical, and the mechanism producing them was
either not active in the 2021 and 2022 campaigns or created shallower events
under our detectability level. 3) The enigmatic events in the K2 data are the
result of an unidentified and infrequent instrumental noise in the original
data set or its data treatment.