Dt21-01:02 discussing the ceremony when an unknown murder
is found states
If a corpse is found slain in the land which the Lord your God gives you to possess, lying in the field, and it is not known who has slain him;
Then your elders and your judges shall come forth, and they shall measure to the cities which are around him who is slain;
Rashi makes the obvious comment that Measurement requires a
to and from. The verse only gives the to part of the
measurement. The from part of the request is from the
Advanced Rashi: Rambam gives more detail (Laws of Murder Chapter 9).
If say the corpse is decapitated - the head and body are separated - then you measure from the nose.
My opinion is that Rambam gave the details of the laws. By contrast, Rashi
spoke more to the exegesis of the text. The verses mention corpse and
therefore the missing to refers to the corpse with which the
verses introduces. The reason Rashi did not go into further detail is because this further detail - nose of the corpse - cannot be derived from the verses being studied.