Measurements of magnetic fields in solar prominences via Hanle effect usually assume either single scattering approximation or simple, one-dimensional, slab model in order to perform an inversion and find the unknown magnitude and the orientation of the magnetic field from spectropolarimetric observations. Here we perform self-consistent NLTE modeling of scattering polarization in inhomogeneous 2D slab, illuminated from its sides by the solar continuum radiation. We show that even in the absence of a magnetic field, in the non-optically thin regime, significant non-zero Stokes U is to be expected. Neglecting these effects, in principle, could cause systematic errors in spectropolarimetric inversions, in the case when the prominence is optically thick.