We set up a tree-level six point scattering
process in which two strings are separated longitudinally such that they could
only interact directly via the non-local spreading effect predicted by light
cone gauge calculations and the Gross-Mende saddle point. One string, the `detector', is
produced at a finite time with energy $E$ by an auxiliary $2\to 2$ sub-process,
with kinematics such that it has sufficient resolution to detect the
longitudinal spreading of an additional incoming string, the `source'. We test
this hypothesis, and find that in this regime, the amplitude exhibits support
over the predicted longitudinal separation of order $\alpha'E$, with its peak
support shifted early by this amount.

In an appropriate kinematic regime, the worldsheet OPE leads to a factorized
Regge form for the amplitude which precisely corresponds to the decomposition
into the auxilliary four-point process and the detector-source
scattering. We support this interpretation by showing that the size of
the amplitude is inconsistent with other interactions between the incoming
strings, that the effect is robust against deformations that preserve the
predicted spreading, and that the amplitude degrades as predicted when deformed
so that the resolution of the detector string is reduced. We
contrast this with the behavior of tree-level quantum field theory with similar
kinematics. This provides a direct manifestation of the scale of
longitudinal spreading in a gauge-invariant S-matrix amplitude, in a calculable
process with significant amplitude which simulates the dynamics of
time-translated horizon infallers.