  Recent detections of merging black holes allow observational tests of the
nature of these objects. In some proposed models, non-trivial structure at or
near the black hole horizon could lead to echo signals in gravitational wave
data. Recently, Abedi et al. claimed tentative evidence for repeating damped
echo signals following the gravitational-wave signals of the binary black hole
merger events recorded in the first observational period of the Advanced LIGO
interferometers. We reanalyse the same data, addressing some of the
shortcomings of their method using more background data and a modified
procedure. We find a reduced statistical significance for the claims of
evidence for echoes, calculating increased p-values for the null hypothesis of
echo-free noise. The reduced significance is entirely consistent with noise,
and so we conclude that the analysis of Abedi et al. does not provide any
observational evidence for the existence of Planck-scale structure at black
hole horizons.
