Got a VO2 Max reading from cycling but don't have a power meter. How?

As you assumed, your fenix doesn't have the necessary information from your cycling to calculate VO2max. 

Your Garmin fenix can calculate VO2max during automatically detected walks that meet minimum threshold requirements (duration and intensity). The reliability of fitness detection during these kind of walks are good and generally usable, but not as reliable as what you can get by recording a a run with GPS. So, these automatically detected fitness values from walking are typically disregarded, giving preference to recorded runs. 

That said, having at least some idea of your VO2max fitness level is useful and valuable - compared to having no idea. So, if your Fenix gets thirsty enough it will display a VO2max calculated automatically during a walk. This value, however, is never fed into Training Status. 

Update: spoke with Garmin and they're piloting a feature to allow VO2 max calculation for cycling without a power meter. Sounds like early days and they're still calibrating, but explains why I see that score!

Never say never, but I would be very surprised if anything like what you describe is being publicly tested out in the wild. 

I think HermanB is correct (and he's from FirstBeat, so he would know).

Similarly, I recently got a Lactate Threshold metric on my watch, without ever doing a Lactate Threshold Guided Test activity. I'm assuming it's because a particular run I did (with a chest strap) happened to coincidentally meet the requirements for collecting Lactate Threshold data, even if it wasn't actually a guided test.

Just telling you what Garmin Support told me - that they are piloting a VO2 Max feature for cycling without a power meter, and that they figure I'm in the test group. Feel free to ask yourself and see if you get a different answer.

This was strange enough to try and track down. But I can say with a high degree of certainty that you got some bad information from Garmin Support. It is possible that the Updated VO2max screen is popping up at the end of a cycling activity based on an unanticipated sequence of information availability.

But a VO2max was not calculated during a cycling activity recorded without a power meter.

Being able to reliably detect fitness during cycling without the need for an expensive piece of specialty equipment like a power meter would be great, and I'm positive that this is being discussed, explored and studied at some level, somewhere - so it wouldn't be difficult for someone in support to venture this working as a possibility. 

C.sco said:

Similarly, I recently got a Lactate Threshold metric on my watch, without ever doing a Lactate Threshold Guided Test activity.

Yes, that's standard behaviour. You get lactate threshold if You run near or above it. I never start separate test, only get this while I run faster. VO2 max I get in every run with chest strap.

Any news? 

I have one of these as well. has not updated since July! But is there. No power metre yet either.