  -- Context. A recently discovered thin long object aligned with a nearby
galaxy could be the stellar wake induced by the passage of a supermassive black
hole (SMBH) kicked out from the nearby galaxy by the slingshot effect of a
three-body encounter of SMBHs. Alternatively, the object could be a bulgeless
edge-on galaxy coincidentally aligned with a second nearby companion. In
contrast with the latter, the SMBH interpretation requires a number of unlikely
events to happen simultaneously. -- Aims. We aim to assign a probability of
occurrence to the two competing scenarios. -- Methods. The probability that the
SMBH passage leaves a trace of stars is factorized as the product of the
probabilities of all the independent events required for this to happen
(PSMBH). Then, each factor is estimated individually. The same exercise is
repeated with the edge-on galaxy interpretation (Pgalax). -- Results. Our
estimate yields log(Pgalax/PSMBH) simeq 11.4 pm 1.6, where the error is
evaluated considering that both Pgalax and PSMBH are products of a large number
of random independent variables. Based on the estimated probabilities, PSMBH <
6 x 10**-17 and Pgalax > 1.4 x 10**-5, we determined the number of objects to
be expected in various existing, ongoing, and forthcoming surveys, as well as
among all observable galaxies (i.e., when observing between 10**6 and 2 x
10**12 galaxies). In the edge-on galaxy scenario, there are always objects to
be detected, whereas in the SMBH scenario, the expectation is always compatible
with zero. -- Conclusions. Despite the appeal of the runaway SMBH explanation,
arguments based on the Occam's razor clearly favor the bulgeless edge-on galaxy
interpretation. Our work does not rule out the existence of runaway SMBHs
leaving stellar trails. It tells that the vD23 object is more likely to be a
bulgeless edge-on galaxy.
