One of the achievements in the Steam release of Black Mesa is to bring a "Hidden Hat" to Xen. This achievement bears the picture of these purple hats.
Co-ordinates
Hat 1: map bm_c3a2g -7, 2319, 544
Hat 2: map bm_c3a2c -792, 2943, 336
Hat 3: map bm_c3a2a 4594, -1440, -2237
Collected by DeathTBO
Purple hat 1
Purple hat 2
Purple hat 3