Could it be a file extension that is tripping autoplay?
Well, MC has a file association for Blu-ray discs. But it isn't a simple issue.
The actual files on the disc that gets referenced in a log, and actually exists, is for example "D:\BDMV\index.bdmv". But MC references the disc in the CD Database of the library as "snatch\BDMV\index.bluray;1", while the file association in MC options refers to "bdmv,mpls" for Blu-ray, which correctly reflects the file type being played, and the playlists associated with the movie.
So my answer to that question is; I have no idea. But the files association Playback Method is set to "Automatic", so maybe MC does just take that as an instruction to play the title, even though the MC Autoplay settings say to do nothing. But file associations are supposed to enable double-clicking a file in Windows Explorer and having the correct program open the file. They shouldn't be defining what happens inside MC.
Interestingly, if I changed the Playback Method in the File Association setting in MC to "External Program (default)", then reinserted the blu-ray disc, I received a Windows popup message asking to confirm which program to use to play the disc. This would imply that it was Windows trying to play the disc using its File Associations.
[EDIT: Actually, this was Windows responding to the MC request to play the bdmv file using the Windows Default player.] Disappointingly it was defaulting to MC23! I haven't uninstalled that from my HTPC yet, but I did go through the Windows File Associations and Default Programs and set them all to MC24. Sign. That continues to be an issue.
I mention the Windows File Association still pointing back at MC23 because after I changed them all to MC24 in Windows, so that MC23 was no longer associated with any file type (I'm sure I had done that before - time to uninstall MC23 to stop any possible interference) I got a different result from earlier. Initially, I got a mostly blank screen with the JRiver logo in the center. So I figured a reboot was required.
After the reboot, the first time I waited long enough and nothing happened, but then when I pressed an arrow key on the keyboard to move within the Video menu, the movie started playing.
When I tried that a second time with the "Video" menu item highlighted, and pressed the down arrow to check in the Blu-ray item was present, the disc didn't start playing and the Blu-ray item was present. So an inconsistent result, with the differences being one test was immediately after a reboot, and I moved within the Theatre View menu structure differently.
So I uninstalled all earlier versions of MC (MC23 32 & 64 bit), rebooted, reinstalled MC24 latest using the Custom installation, and selected all file types so that MC24 had the association for all file types it can open, then rebooted. Then check all the Windows File Associations which seemed okay. Specifically the bdmv file type was pointing to MC24.
Finally, I inserted the same disc I had been testing with and almost immediately got the message in the attached image. I took that to mean the MC24 tried to scan/identify/open/play the disc. Probably just scan. But it couldn't because DVDfab was working on it still. I left the message for some time, and then selected 'Yes" to try again. The movie didn't play, and the Blu-ray menu option was visible in Theatre View.
So then I inserted a new Blu-ray disc that MC24 and DVDfab have never seen. DVDfab did its work, but the Theatre View menu stayed sitting on "Guide" and the Blu-ray menu item didn't appear. Eventually I pressed the Up arrow to move the highlight to the "video" menu heading, and the Blu-ray menu item appeared. The movie didn't play.
TL;DR
If a Blu-ray (and probably other optical discs) play when inserted, even though everything is set to do nothing, check if the File Association for that disc type (bdmv for Blu-rays) is pointing to the correct version of MC.
I don't know if this is actually the answer, because I got pretty inconsistent results, but for the moment it seems to be making a difference.
PS: I have one log from earlier testing, and could make another after doing all the above, if anyone is interested.