For the purpose of my Kabal of Agony experiment I've reviewed the slave capture rules, and the various FAQs and will use the following to determine prisoner capture.
If the Dark Eldar win a close combat, and the opponent is forced to fall back, each model killed in that combat is captured on a 4+. If the squad is caught in a sweeping advance, then all models destroyed this way are captured.
In the case where the enemy squad is destroyed at the end of the assault phase, any models killed during the close combat will be captured on a 4+. This will include any loses due to no retreat, or self inflicted wounds such as perils of the warp, bosspoles, etc.