Most likely it's the rear seal in the master cylinder between the power brake booster and master cyl.
If not power brakes then between the master and firewall.
Either way it's got to come off.
Could just be the cover seal though. (pics are worth,,,,)
Ah, another one of those we can't see for ourselves. (Bummer)
Oh, and one note to mention.
I have been known to taste fluid from under a car just to see exactly what it is, and brake fluid is REALLY bitter.
Just spit it out a few times.
Motor oil just tastes like oil and not bitter.