  We point out a new mechanism giving rise to anomalous tau neutrino appearance
at the near detectors of beam-focused neutrino experiments, without extending
the neutrino sector. The charged mesons ($\pi^\pm, K^\pm$) produced and focused
in the target-horn system can decay to a (neutrino-philic) light mediator via
the helicity-unsuppressed three-body decays. If such a mediator carries
non-vanishing hadronic couplings, it can also be produced via the
bremsstrahlung of the incident proton beam. The subsequent decay of the
mediator to a tau neutrino pair results in tau neutrino detection at the near
detectors, which is unexpected under the standard three-flavor neutrino
oscillation paradigm. We argue that the signal flux from the charged meson
decays can be significant enough to discover the light mediator signal at the
on-axis liquid-argon near detector of the DUNE experiment, due to the focusing
of charged mesons. In addition, we show that ICARUS-NuMI, an off-axis near
detector of the NuMI beam, as well as DUNE, can observe a handful of tau
neutrino events induced by beam-proton bremsstrahlung.
