  Strong, globally-organized magnetic fields are found for a small fraction of
O, B, and A stars. At the same time, many theoretical and indirect
observational studies suggested ubiquitous presence of weak localized magnetic
fields at the surfaces of massive stars. However, no direct detections of such
fields have been reported yet. We have carried out the first comprehensive
theoretical investigation of the spectropolarimetric observational signatures
of the structured magnetic fields. These calculations are applied to interpret
null results of the recent magnetic surveys of massive stars. The intensity and
circular polarization spectra of early-type stars are simulated using detailed
polarized radiative transfer calculations with LTE model atmospheres. Similar
to observational analyses, the mean Stokes I and V line profiles are obtained
by applying a multi-line averaging technique. Different spectropolarimetric
observables are examined for multiple realizations of randomly distributed
radial magnetic field spots with different spatial scales. We characterize the
amplitude of the circular polarization profiles and the mean longitudinal
magnetic field as a function of magnetic spot sizes. The dependence of these
observables on the effective temperature, projected rotational velocity, and
inclination angle is also investigated. Using results of the recently completed
Magnetism in Massive Stars (MiMeS) survey, we derive upper limits on the
small-scale magnetic fields compatible with the MiMeS non-detections. According
to our simulations, existing spectropolarimetric observations of sharp-lined
massive stars rule out the presence of the small-scale fields stronger than
50-250 G, depending on the typical spot sizes. For broad-lined stars, the
observations constrain such fields to be below approximately 1 kG.
