Idle HomePod transmitting GBs of data daily

The culprit has been found. There's a Mac mini on my LAN. It's decided that it really really needs to communicate with the HomePod. Using Wireshark, I've traced the traffic between the two. Every 10 seconds, the Mac mini issues 5,000 or so connections to the HomePod. It does this over a couple of seconds. After each TCP connection is established (with a 3-way handshake), the Mac mini sends a GetInfo Airplay query to the HomePod. The HomePod responds with two packets: one containing RTSP/1.0 200-OK and several lines of text with an x-application-plist (1514 bytes) and a second containing info regarding Audio Accessory (328 bytes).

The HomePod sends back the requested information after receiving each GetInfo query from the Mac mini. The total conversation between the two comprises 14 packets.

The Mac mini floods the HomePod with thousands of these connections over 2 to 3 seconds. It then backs off, and does the same thing, every 10 seconds. All this AirPlay handshaking traffic between the two, initiated by the Mac mini, is the cause of the multi-gigabytes of WiFi traffic going to and from the HomePod.

Rebooting the Mac mini stopped the flood. I don't know what might of triggered this aberrant behavior by the Mac mini. I'll have to keep an eye on the traffic stats reported by the Unifi AP to see if it recurs.

FYI - it's been about a day since I restarted the Mac, which returned HomePod network traffic to normal. Since then, the telemetry on the Unifi AP shows about 35 MB up and down. That's entirely background housekeeping/handshaking traffic, as the HomePod has been sitting dormant (no Hey Siris). I'm not sure if Unifi counts all broadcast packets on the LAN that get sent to the HomePod, or just unicast to/from the HomePod. Probably just unicast, but who knows.

I've done a little troubleshooting, but no insights have emerged. I rebooted the HomePod (power cycled), but it continued to transmit gigabits each day. Then I tried Forgetting it (ie, remove it for the Home app), and then set it up as a new device. No change - yesterday it transmitted about 5 GB to my WiLAN, and downloaded about 2 GB.

I can't really tell if these excessive data flows are to/from the internet or are purely local. I've looked at the stats in my router. We have a fair bit of daily internet traffic logged, but it's mostly downloads (Netflix, Crave, YouTube, etc). I don't see signs of gigabytes of daily outbound traffic.

Today's experiment - I used the 'family' controls in my router to block any internet access by the HomePod. If the HomePod data flows are coming and going through the WAN interface, they should now come to a halt.

I think it’s a waste of bits (and my time) to send feedback to Apple:

1) they’ve discontinued the original HomePod. 

2) they no longer support El Capitan (which is what’s on my Mac mini). Nor Server 5.2, for that matter. 

Given the above, I think the likelihood that they would read (and take action on) any feedback about obsolete products is zilch.

Strange thing - I never used AirPlay on the Mac mini. Although it’s discovered both the Homepod and my ATV as available AirPlay devices, I’ve never selected either of them for audio output.  The trigger that caused the Mac mini to start hammering on the HomePod is a mystery. 

Update from OP:

Since restarting the Mac mini three weeks ago, the problem has not recurred.

Telemetry from the Unifi AP indicates that over the last 21 days, it has sent 751 MB to the HomePod, and received 676 MB.

There were a few "Hey Siri, what's the weather?" queries, which accounts for the slightly larger downstream traffic. But the majority of the traffic is background iOS/Airplay housekeeping.