  The declining lightcurve of the optical afterglow of gamma-ray burst
GRB000301C showed rapid variability with one particularly bright feature at
about t-t0=3.8 days. This event was interpreted as gravitational microlensing
by Garnavich, Loeb & Stanek (2000) and subsequently used to derive constraints
on the structure of the GRB optical afterglow. In this paper, we use these
structural parameters to calculate the probability of such a microlensing event
in a realistic scenario, where all compact objects in the universe are
associated with observable galaxies. For GRB000301C at a redshift of z=2.04,
the `a posteriori' probability for a microlensing event with an amplitude of
dm>=0.95 mag (as observed) is 0.7% (2.7%) for the most plausible scenario of a
flat Lambda-dominated FRW universe with Omega_m=0.3 and a fraction f_*=0.2
(1.0) of dark-matter in the form of compact objects. If we lower the
magnification threshold to dm>=0.10 mag, the probabilities for microlensing
events of GRB afterglows increase to 17% (57%). We emphasise that this low
probability for a microlensing signature of almost a magnitude does not exclude
that the observed event in the afterglow lightcurve of GRB000301C was caused by
microlensing, especially in light of the fact that a galaxy was found within 2
arcsec from the GRB. In that case, however, a more robust upper limit on the `a
posteriori' probability of ~5% is found. It does show, however, that it will
not be easy to create a large sample of strong GRB afterglow microlensing
events for statistical studies of their physical conditions on micro-arcsec
scales.
