How do I find the update log for this game (to see what changes have been made to put it in the "updates" section of my games)?

The "Changelog" is a relatively recent addition to GOG. I don't believe any prior changes were brought forward, so it only records updates after the introduction of the Changelog.

Prior updates were normally announced with sticky threads in each game's subforum, and a post in the "What did just update thread".

So, from the look of things, BaSS has only had one update: the addition of a Security Manual bonus goodie (May 16, 2014)