The late assembly of massive galaxies is thought to be dominated by stellar accretion in their outskirts(beyond 2 effective radii Re) due to dry, minor galaxy mergers. We use observations of 1010 passiveearly-type galaxies (ETGs) within z < 0.15 from SDSS IV MaNGA to search for evidence of thisaccretion. The outputs from the stellar population fitting codes FIREFLY, pPXF, and Prospectorare compared to control for systematic errors in stellar metallicity (Z) estimation. We find thatthe average radial logZ/Z⊙ profiles of ETGs in various stellar mass (M*) bins are not linear. As aresult, these profiles are poorly characterized by a single gradient value, explaining why weak trendsreported in previous work can be dicult to interpret. Instead, we examine the full radial extent ofstellar metallicity profiles and find them to flatten in the outskirts of M* ≳ 1011M⊙ ETGs. This is asignature of stellar accretion. Based on a toy model for stellar metallicity profiles, we infer the ex-situstellar mass fraction in ETGs as a function of M* and galactocentric radius. We find that ex-situ starsat R∼2Re make up 20% of the projected stellar mass of M* ≲ 1010.5M⊙ ETGs, rising up to 80% forM* ≳ 1011.5M⊙ ETGs.