Yamaha AVR, not Aventage, 5 years old
Samsung 75” 85 series QLED
Comcast box, not 4k
URC MX890/MRF260
Metra HDMI hot plug interruptor between AVR and 20’ HDMI to TV, settled down TV blue screen issues

Less relevant gear:
Yamaha BDP
Ubiquiti WiFi in same cabinet
Comcast gateway and Netgear switch there also

Audio turned on at 5AM three nights ago
OK two nights ago
Audio again last night 3:30AM
TV does not come on

Today:
I made sure HDMI CEC was off in AVR and TV
Disconnected ethernet from AVR
Reset network settings on TV so it’s not on the network now either
Tried to eliminate anything else in menus that could be a trigger

For test, I am having the client remove the battery from the MX890 before bed to just eliminate that from the mix.  If we get a week or two of stability we can leave it in all the time.

I’ve not run into this in 30+ years. Has anyone seen this or know of anything I may have missed?

Yikes. Does it turn on to the source it was last set to, or does it switch to video?

If you want a work around IF it turns on to the last source, build the turn off macro to switch to an unused input on the Yamaha as the step before powering down the Yamaha....

Has it ever randomly turned on before during the day? Do they have a cat who discovered the joys of watching TV in the middle of the night?

Cats can be persistent.
[Link: kcci.com]

If Network Standby has been selected, turn it off. I had the same problem with one client's Denon AVR and this solved it.

i did turn that off yesterday.

Nomcats, other than Cat5 😀

So far just the two night time events, nothing during the day. The only used source is the Comcast box and that is where the sound came from. Your workaround is a good idea for added security.

No phone call yet today.

Remember to modify the power on command to select the cable box input..

Neighbor with URC on same ID?

I was going to type the same thing as musictoo "Neighbor with URC on same ID?"

To add to that, we all know how URC's base stations are not very good at blocking crosstalk IDs sometimes.

The solution to this one might be to use the "i" version of the MX-890 and MRF260 (433MHz instead of the standard 418MHz). This assumes that a neighbor with a URC remote is actually the problem.

ghostbusters...

Turning OFF CEC would be my suggestion.

I had a recent case where a Samsung TV power command would trigger a SONY TV in another room via CEC connected through a Blue-Ray player, connected to an HDMI splitter. I used a CEC blocker to the Samsung TV. There is no control system installed. It was very frustrating because only the Samsung Power ON command was forwarded, Samsung Power OFF had no effect on the Sony TV.

Originally, the system was fine. I suspect that an update of something enabled the CEC mess -- likely the Sony TV. I had other connectivity issues with that TV that changed over time and seemed to be CEC related.

I've had issues with Comcast cable box updates turning things ON in the middle of the night. Note that you can schedule Comcast updates. It would pay to check if the cable box update time window correlates with any of these events.

Make sure CEC is off in the cable box as well. This fixed it for us

I know you said "ethernet" was unplugged, any chance WiFi is on and feeding the receiver internet?

I had a very similar situation with a Yamaha receiver... Turned out to be the customer's kid streaming Spotify connect to the wrong destination. Kid was at college, tried to stream Spotify to her family room but was actually streaming to her parents family room.

Simple solution was to disconnect the receiver from the internet. The parents weren't using any internet features, so no big deal turning that off.

Problem solved.

Schedule is in cable box settings? And CEC in Comcast settings?  I don’t recall seeing either, but can look.

Don’t think it’s a neighbor, 1 acre lots, too far away. And 3:30AM?

It was never set up for WiFi, wired only, so no WiFi credentials ever entered in.