Battery discharge while Macbook was turned off in 1 week. Possible causes?

I brought my MacBook Pro with Apple Silicon about 1 month ago, and I'm using it to work on pet projects occasionally, but not daily. I left it unused for one week and when I turned it on it wasn't starting, as the battery was drained (the screen indicator was showing).

AFAIK laptops should not discharge entirely after a mere 1 week of no use. Charging it back went fine, the OS is reporting the battery health is good, coconutBattery was also showing that everything is ok, the battery has 3 cycles so far. I ran last shutdown in the terminal to double-check shutdowns and exclude the possibility that I put it to sleep instead. I also rand Apple Diagnostics and no problem was found, so I'd assume it's not the logic board or any other hardware. After charging I also did a quick test to see how much it discharges in about 2 hrs and then after 4 hrs, but it was still at 100%.

I've stored the notebook in a drawer at room temperature (~22°C). So, if the diagnostics are real, I can't think of what could've drained my battery. Any ideas?

EDIT: New idea that came to me: could it be that I closed the lid too fast after selecting shut down, consequently making it sleep instead or not turning off some hardware properly?

EDIT #2: I ended up investigating logs with log show --style syslog. There are some SMC errors saying something like "could not write SMC key". I'm quite new to innards of MacBook, but this sounds to me like it couldn't shut down properly. That would also explain why there are syslog entries from hours after I've shut down the computer and also from the following day.

EDIT #3: About 24hrs. since last recharge and I've switched it on/off several times, and also used it a bit (cumulated usage about 1h), and the battery just recently dropped to 99%. So I'd rule out battery defects and say it was probably a shutdown failure (maybe I closed the lid too fast or maybe some process was blocking the shutdown).

Solution 1:

These terminal commands will show sleep/wake activity since the MacBook was last powered up.

pmset -g log | grep -e "Wake from" -e "DarkWake" -e "due to"
pmset -g log | egrep "\b(Sleep|Wake*|DarkWake|Start)\s{2,}"

They both extract data from the power management log and produce very similar output. Try one or other (or both). The output includes the battery charge level.

You may be surprised at the number of times the MacBook at least partially/briefly wakes whilst you thought it was firmly asleep.