Unknown Upload for Days

Hi folks. Here's one for ya.

I have a server that's been spewing packets for a handful of days now, at over 300 Kbps. I can't figure out what's doing it. I've narrowed it down to that machine. Server.app demonstrates there's been significant outgoing traffic to the Internet side of my authenticating router. Turning off web doesn't fix this.

No major changes on that machine recently. Software Update is off (has been for some time), even though that's incoming.

Any advice as to how I can see what's doing this?

Cheers