Context. 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 have suggested the ubiquitous presence of weak localized magnetic fields at the surfaces of massive stars. However, no direct detections of such fields have been reported yet. Aims. 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. Methods. The intensity and circular polarization spectra of early-type stars were simulated using detailed polarized radiative transfer calculations with LTE model atmospheres. Similar to observational analyses, the mean Stokes I and V line profiles were obtained by applying a multiline averaging technique. Different spectropolarimetric observables were examined for multiple realizations of randomly distributed radial magnetic field spots on different spatial scales. Results. 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 nondetections. Conclusions. According to our simulations, existing spectropolarimetric observations of sharp-lined massive stars rule out the presence of any small-scale fields stronger than 50–250 G, depending on the typical spot sizes. For broad-lined stars, the observations constrain such fields to being below approximately 1 kG.
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