TT #335 · Parashas Shoftim · Seventh Aliyah · Passuk 21:01
If a slain person is found in the land that the Eternal your God is giving you to possess, lying in the field, and it is not known who struck him.
Torah Temimah points to Sotah 44b.
TT lemma: כִּי יִמָּצֵא.
English: ‘If it is found’ – excluding one that was already known from the beginning.
Derash on the discovery (Bavli) — the law applies to a newly found body.
The rite is for a body that is discovered; one whose presence was already known does not initiate the procedure.