Extended tracking busted on InReach mini with 3.70?

Are you sure you enabled extended tracking? 

Hourly message checks are disabled in extended tracking mode. The only message checks would be piggy-backed on the acknowledgements for sent track points. Detailed track logs are also disabled in this mode. 
https://www8.garmin.com/manuals/webhelp/inreachmini/EN-US/GUID-089C5143-55A5-4764-95ED-2EA71DBC21D7.html

I have not tried this lately. That said, I also never trust the marketing hype.

yep - definitely enabled extended tracking (that's also what the icon in the bar means).

But your comment about hourly message checks being disabled also points to a clue on the possible bug:  When I brought it inside after it's three day test, for it's last check-in for messages it said "43 minutes ago" - which makes me think it didn't properly disable the hourly check.

I'm currently doing another three day soak with extended tracking off and not finished yet, but it looks to me (at least on my unit) - it will end with exactly the same power draw as "extended tracking".  Which sure makes me think extended tracking is busted.

Would you be willing to try putting your unit outside with extended tracking on (and no location updates enabled) and see what you see?  I suspect it will be a lot easier to convince garmin to investigate this bug if we could report "two people tried this with two different units and saw the same thing".  However, if your unit didn't show this problem after threeish days with 3.70, it would point to my unit being somehow defective (which I think is unlikely, but TBD).  If you don't have time to try this though no biggy - I understand.

(Agreed.  I also don't trust marketing hype - which is why I wanted to test this at home before depending on it)

One thing you might try is a factory reset. This is a big hammer, but sometimes "fixes" glitches that are due to weird device state. If you do this, be aware that you will lose all user data on the device. And you will have to repeat the outdoor portions of the activation.

good idea.  I'll try that next.

An update on my testing: I just completed a 3 day soak with extended tracking off (but otherwise the same test conditions).  Battery drain was essentially identical, with 44% remaining.

Ok.  I did some more investigation (mostly a series of multiday outdoor tests).  Alas, factory reset didn't help.

But I think I now see what the bug is (and it is fairly minor): The extended tracking feature (regardless of what it says in the manual or the status bar) doesn't actually activate until you've turned on automated location sending.  If you leave that off (like I was hoping to do, because I don't care about automated tracking but I do want 20 day battery life) - the power draw is identical to the non extended tracking case.

So the workaround that will be fine for me: I'm just going to keep this device off except a few times a day when I want to send a preset message (with location) to my wife.

But in case it is useful, here's my full notes - including 'real world' power costs of ANT+ (minor) or BLE link (more substantial).

Summary: (assuming no location tracking).
* ANT & BLE off, about 5.5 days of battery capacity
* Leaving ANT+ on should allow about 4.5 days of battery capacity
* Leaving BLE on should allow about 4 days of battery capacity

11/27 1pm:
* did "reset to defaults"
* reactivated with service
* paired bluetooth with my phone and used garmin app to "sync" inreach
* set outside 20 mins
* send "test" message and confirm service is working
* turn off device and charge to 100% (and allow to sit charging for a few hours after it said 100%)

11/27 3:42 pm:
* turn on device
* turn off bluetooth
* set tracking send interval to 4 hrs (to create lowest possible power drain, even though in these tests tracking is never even started)
* place outside, clear view of sky

11/30 12:46
* battery has decreased to 45% in three days on "extended tracking" - far below the 20 days in the doc

Theory on bug: extended tracking (other than disabling bluetooth & ant+) doesn't take effect UNTIL you turn on sending location updates. So it is not a good option if you just want to keep the device
on.

11/30 3:30 pm:
* Turn ext tracking off, but ANT+ on (with no ANT client connected). Start outside soak, I'm assuming it will burn all but about 45% of battery in three days.

12/3 2:42 pm:
* At 37%, so about 10%ish higher due to ANT+. So 3-4% per day if you leave ANT+ on.

12/3 4:45 pm:
* Started 100% full test outside, ANT+ off, BLE on. BLE occasionally connected to my phone in the house.

12/4 3:00 pm:
* After 23 hours, the BLE on, ANT off test shows 77% battery remaining. So that looks like BLE would last about 4 days