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My 2008 hybrid has been sitting idle for about 5 days, and before then, only did short (5-10 miles) trips every 2-3 days since the COVID lockdown started. Last night, I tried to start it and the engine did not turn. These are the details:

-The lights flashed and remained on
-IMA meter went up to about 1/3 full
-When I turned the key, it gave two weak chug sounds and then stopped. It did NOT do the traditional clicking noise you get with a dead battery.
-The only thing I did differently the last time I went out was that I had it running idle for about 30 minutes waiting for a pickup. During that time, I was using a fast charger in the lighter socket for my phone. Not sure if that impacts things but figured I'd list all variables.

This morning, I used my plug-in battery charger/tester. The tester said the main battery was at about 75% (which makes sense since it was replaced about 2 years ago). It gave an error when it did an alternator test, though it recommended 10 minutes of idling before testing the alternator which I couldn't do since I couldn't start the car! I did the jump-start function on there, and then the car started. When it started, the IMA was still at about 1/3. Drove it for about 30 minutes on the highway very conservatively, so the IMA charge hit full pretty quickly.

Also, the IMA battery was replaced within warranty in 2015. Barely within warranty but still so I got it replaced.

Does this indicate anything other than the standard battery was unused for too long? And if that's the case, what's the ideal amount of usage during this time when I don't really have to drive to work.