A couple of times I've gotten an alert that says "Your free human zap (buff) expired :(" but I never noticed anything saying that I got that. If it happened again, where would I see it?

Oh this actually happened to me several times and but i never figured out, how to solve this issue myself!

pinging to come back here and check possible responses

I don't think that it ever worked properly? I also only had the expiration notice once or twice, but never a notice when actually getting a free zap.

Ah, that's odd, good to know it's not just me though.