Kernel Density Estimation (KDE) of the ASPCAP-derived [Si/Fe] abundances for MW stars, as a function of [Fe/H], with contours showing the density of stars that appear to be a metal-poor extension of the thick disk and halo locus. These contours demonstrate that there is a low-density boundary (light green thick lines) separating two higher density regions, one with lower [Si/Fe] (black contours), and one with higher [Si/Fe] (light green shadow region). The BACCHUS-derived [Si/Fe] abundance for the newly identified Si-enhanced stars is highlighted with a black empty square (undetermined [Al/Fe]), red empty diamonds ([Al/Fe] ≲ +0.5 in both the intermediate- and low-aluminum regimes, which are likely to be chemically tagged as escaped former members of MW dwarf spheroidal satellites or GCs), and red filled diamonds (with [Al/Fe] ≳ +0.5 or Al-enhanced stars, which are likely to be chemically tagged as migrants from GCs). Over-plotted are data from Fernández-Trincado et al. (2016b, 2017, 2019b; empty black triangles), from Fernández-Trincado et al. (2019c; empty black circles), and GC stars from Mészáros et al. (2020; gray dots and empty blue star symbols–highlighting the ω Centauri population). The plotted error bars show the median abundance uncertainty from BACCHUS. The number of APOGEE-2+ stars each bin are shown at the center of the bin at the bottom of the plot.