  The possible detection of echoes in late gravitational-wave signals is the
most promising way to test horizonless alternatives to general relativistic
black holes, and probe the physics of these hypothetical ultra-compact objects.
While there is currently no evidence for the presence of such signatures,
better accuracy is expected with the growing wealth of data from gravitational
waves observatories. So far, several searches for these specific signals have
been performed considering equidistant intervals between consecutive echoes,
i.e. quasi-periodic wave-forms, and ignoring possible backreaction effects of
the incoming waves. Here we study scalar perturbations in exotic compact object
scenarios that account for possible backreaction phenomena. In particular, we
find that if one considers the increase of the central object mass due to the
partial absorption of the energy carried by the perturbation, the echo signal
can be quite different and non-periodic. Apart from this simple scenario, we
also consider the case in which, in order to preserve its compactness above the
black hole limit, the compact object absorption shuts down in a finite amount
of time or leads to an expansion. In both these cases we find interesting new
features that should be taken into account in future searches.
