Magnesium-to-iron ratio [Mg/Fe] against calcium-to-iron ratio [Ca/Fe]. Mg-poor stars are identified with colored points. The two red Mg-poor stars present [Mg/Ca] ≲ −0.3 and [Na/Fe] < 0, which is consistent with pollution of their star-forming ISM from SNe Ia (I. I. Ivans et al. 2003). The low occurrence of these signatures (2/75) is expected given how signatures of individual pollution events smear out at [Fe/H] ∼ −2 (T. T. Hansen et al. 2018). The magenta point marks a Mg-poor star that shows dynamical traces of being part of an accreted system and abundances consistent with a UFD (B. Letarte et al. 2010; A. Frebel et al. 2014; A. P. Ji et al. 2019; H. D. Andales et al. 2024).