  The common observations of multiple radio VLBI stationary knots in
high-frequency-peaked BL Lacs (HBLs) can be interpreted as multiple
recollimation shocks accelerating particles along jets. This approach can
resolve the so-called "bulk Lorentz factor crisis" of sources with high Lorentz
factor, deduced from maximum gamma-gamma opacity and fast variability, and
apparently inconsistent slow/stationary radio knots. It also suggests that a
unique pattern of the non-thermal emission variability should appear after each
strong flare. Taking advantage of the 13 years of observation of the HBL Mrk
421 by the X-ray Telescope on the Neil Gehrels Swift Observatory Swift-XRT, we
probe for such an intrinsic variability pattern. Its significance is then
statistically estimated via comparisons with numerous similar simulated
lightcurves. A suggested variability pattern is identified, consistent with a
main flare emission zone located in the most upstream 15.3 GHz radio knot at
0.38 mas from the core. Subsequent flux excesses in the lightcurve are
consistent with a perturbation crossing all the downstream radio knots with a
constant apparent speed of 45 c. The significance of the observed variability
pattern not arising from stochastic processes is found above 3 standard
deviations, opening a promising path for further investigations in other
blazars and with other energy bands. In addition to highlight the role of
stationary radio knots as high-energy particle accelerators in jets, the
developed method allows estimates of the apparent speed and size of a jet
perturbation without the need to directly observe any motion in jets.
