Another point I find interesting:
The Detect Life spell (pg. 286-287) explicitly does not detect spirits. Implicitly, this probably also means Homunculi and Watchers since they're not technically spirits, they do work like them in many ways.
Since the magic that "detects life" doesn't recognize spirits as living, arguably a contact trigger wouldn't either.
That's not very PC of Detect Life. Buttercup may press discrimination charges.
Since the exact quote is "... detects all living beings (but not not spirits)...," I think spirits are living beings that are an exception to the parameters of this spell. The use of "but" does not concretely make spirits an exception as opposed to a clarification of the definition of "living beings," but I feel that it is a stronger interpretation that they are living beings that need to be excluded. Why? Maybe to restrict the number of planes that the spell acts on; you can only act on one plane (there are probably exceptions), but if spirits are living beings, they are astral/meta planar living beings and thus may not be detected by a spell focused on the material plane.
Looking at the other detect spells, Detect Enemies does not exclude spirits. Whether they are included as living targets, is up for debate. Detect [Life Form] and Detect [Object] do not use spirit as an example, but that is inconclusive. Detect Magic explicitly detects spirits and not critters or awakened individuals, but it does not do so in the context of living versus nonliving.
Let's change the question a bit and see what you think.
Steve the magician casts detect life. 2 meters to his right, Bob the other magician is slumped over because he is astrally projecting. Bob the other magician's astral form is standing 2 meters to the left of Steve the magician. No one else is in range (assuming we are excluding, mundane insects, bacteria, and other tiny things that are living beings, but would break the spell).
So, how many living beings does Steve the magician detect? One or two? If it's one, does he detect life to the left or to the right?