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
I have heard horror stories on some of the mom subreddits of creeps messing with breastmilk. 

I keep mine in a lunchbox with a label with my name saying "medical, do not remove from fridge" 

I haven't had issues with people touching it.