I've been in contact with said user over their trade, and they definitely were online yesterday...

Is this maybe something that could be happening because they have their "stealth mode" on?

I shouldn't think so, unless this got tampered with. So far even people with status "hidden" would be displayed as "online" or "idle for" on trades. But, come to think of it, they would probably only get an "offline" status and not the 30 days warning, had something been changed in that concern.

This happened to me also