RE: 98 S70

I have 3 sets of keys, 2 with the remote fob, one that is solo with a black
end.

I have never used this key, assuming it's a valet key (car bought used,
btw)...

I went to use it, manually unlocked car, fine.. key in ignition, fine.. turn
to start car.. it turns fine, accessories on, but- alarm sounds.. and car
won't crank or start...

What is the purpose of this key? Worse yet, this is the only spare we've
given to our mechanic, how has he started the car?? I know he's worked on
it... (maybe not?  Just kidding here..)

This is odd...

Thanks!