Garmin Edge 1030 battery drained after I fully charged it and while the device was shutdown

Maybe you have display always on? 

And if device enter in stand by maybe there is an bug?

Also I dont understand why Garmin did not out the logic to have display on only if the activity is started and active, if activity is finished, I dont see the point to have always on in the menu

One of the more-interesting things my 1030 has done is to wake up while being charged. Happens as soon as the computer goes to sleep. The Garmin drops out of charge/download mode and wakes up, and I'm guessing the accelerated drain occurs when it's looking to try and connect with sensors. Probably goes into a high-power xmit/recv mode.

Even more interesting is when it somehow wakes up my bike! The Di2 wireless xmitter can cause the battery to drain very very fast, possibly also going into a high-powered search mode?

This problem comes and goes and comes back again with firmware updates. The moral to the story is, if you charge connected to a computer, unplug the Garmin and turn it off when finished charging.

I just came to a similar root cause.     After the device is fully charged, when I remove the micro USB from the device  it supposedly shuts down.  However, I noticed that the last time I charged and removed the cable,  the screen went blank but the device was not actually shut down.  After charging I will do a manual shut down again to see if that was the problem

It actually doesn't matter whether asleep or fully powered down. It'll randomly and rapidly drain overnight. It could be a combination firmware bug with a combination of hardware revision.  I've had my battery get flattened repeatedly now, which is a horrible thing for a Li-ion battery. When just asleep, normally it'll still be at 90% after months. Now it'll either drain completely overnight, or it'll stay charged forever. 

Only recommendation I have right now is ALWAYS keep it plugged in so it's ready for your next ride. Because I've had it NOT ready a few times now.

I am hoping that you are not correct.  I will test my theory first and hope that I don't run into the problem again.  If it does happen again which means that the device can drain the battery while powered down then there is a big issue that garmin must fix.

I've done two factory resets, to no avail to prevent it.  It's so random, it's hard to catch.  Every time I think it was fixed in the latest update, it comes back right when I'm heading out to ride.  For a pure dose of 2020 irony, it actually seems to happen more often powered off fully than in sleep mode

Yes, I have had this random power-on happen (FW 11.0 on the 1030). For instance, I charged after yesterday's ride with a USB cable on my PC, unplugged it at about 90% battery, and after it booted up, held down the power button and pressed  the power off popup on the screen to turn it off . When I picked it up this morning, the screen instantly came on, and the battery was down to 25%. Exactly the same procedure as I use after every ride, and it usually stays powered off, but maybe half a dozen times I've found the unit has turned itself on overnight.

Yeah, that matches what I'm seeing.  So far no instances of this with the unit in sleep mode in v11.   I _think_ it might have occurred with sleep mode in the 9.8 beta's for 10, or in 10.x  itself, but I definitely know it occurs repeatedly with the unit powered all the way off.

Update time.  I've not had any issues since, and I've not powered it fully off since.  All of the issues I saw appear to have been when I (ironically) powered it all the way OFF.    As long as I don't power it off, the battery doesn't suddenly drain.