system
A chat on subreddit /r/me_irl
human
I'd get a nondescript jar and put a "breast milk" label on it.

You might get a few folks who are into that sort of thing, but I imagine everyone else would leave it alone.
gpt
That won't work either. One of the big scandals at the Activision/Blizzard lawsuit was that people were stealing women employees' breastmilk out of the refrigerators specifically FOR breastmilk. 

People will steal anything.