For those two+ weeks (Dec 4 - Dec 23), the machine was not responding to pings, ssh, or http. I checked from multiple locations in the US and .eu.
Once I rebooted it, I saw from the munin graphs and log entries that the guest VM had been running during those two+ weeks. It was not powered off.
The guest VM logs are filled with messages such as "Dec 4 11:53:38 eu1 ntpd[208]: error resolving pool 0.debian.pool.ntp.org: Temporary failure in name resolution (-3)". The first ntp error log entry coincides with when my external monitors showed the VM stopped responding to pings.
My conclusion is that the guest VM was running, NTP on the guest was constantly complaining about not being able to reach any servers, and the guest was not listening for ping, ssh, http, or https. Seems like a loss of network connectivity is the most likely cause.
I think I'll set a cronjob to do an mtr/traceroute once/hour or so and log it to a file. Maybe that will help to narrow down this network issue next time it goes offline.
Oh, here's an mtr run now that the guest is online again:
5. AS3356 2001:1900:4:3::51 0.0% 10 16.5 17.6 16.2 24.1 2.5
6. AS3356 2001:1900:2::3:12b 0.0% 10 124.5 124.4 124.1 124.5 0.0
7. AS16097 2a02:238::109:104:63:255 0.0% 10 125.2 125.0 124.8 125.3 0.0
8.

9. AS16097 2a02:238:1:f075::1 0.0% 10 134.9 135.1 134.9 135.6 0.0
10. AS35366 2a02:180:6:9::9 0.0% 10 133.3 135.1 133.0 144.9 3.5
11. AS35366 2a02:180:6:5::c 0.0% 10 132.6 132.8 132.6 132.9 0.0
12. AS35366 2a02:180:6:1::4a8 0.0% 10 134.5 134.6 134.4 135.3 0.0
Compared to the mtr run from my initial post, you can see that 2a02:180:6:5::c and the guest, 2a02:180:6:1::4a8, are now responding.