Sudden spike it outbound WAN traffic, even with no devices connected

[ScrewItUpYourself]

This afternoon, I suddenly began having a spike in outbound WAN traffic, creating a noticeable slowdown. The logs are showing anywhere from 20-30 Mbps in outbound traffic. LAN traffic was only a tiny fraction of this. I disconnected all devices for a few minutes, then logged back in to the web interface, and the outbound traffic remained just as high during that period.

Of note, I did upgrade to the latest version a few days ago, but the problem started suddenly this afternoon. Also, I have OPNsense set up to ping Cloudflare's DNS server every 60 seconds to monitor connection quality, and I did note a large amount of packet loss right when the problem began, but that normalized within a couple minutes. My ISP sees nothing on their end, other than the unusual amount of traffic. As soon as I unplug the Ethernet cable from my cable modem to the device running OPNsense, the outbound traffic disappears on their end (so it doesn't sound like it's something spurious being emitted from the modem).

I'm at a loss here--where should I go next with this?