Which BIONICLE set did this mask came from?
I found the answer while I was writing this question, but I'm posting question and answer just for dibs on the bionicle tag ;)

The Peeron page for that mask I just linked to lists the part number as 32571. I do a search for the part number, and come across the Brickipedia page for that part. Scrolling down, I find the source:

Transparent:

8525 Masks (random)
8530 Masks (random)


Apparently, I found this mask along with the numerous bronze and silver Great Kanohi masks in one of those mask packs.
Fun fact: this colorless Kaukau was designed to be wearable by Toa Kopaka, and possibly also the Ko-Matoran. The Toa is known to have had this mask equipped in BIONICLE Chronicles 1: Tale of the Toa, but it's not entirely clear if this is part of the canon. There aren't any named Ko-Matoran with this mask as far as I know.