Well, what was the ambient temperature at the time you got the texts? Very hot those days?
If the battery gets too hot or too cold, it does not perform well and/or may lose charge. The battery temperature is influenced by charging (which causes it to heat up) and the ambient environment. So, if the environment alone becomes too hot or cold, the car may attempt to regulate the battery temperature
at any time, even when not charging and not plugged in. You could also get the same notification from the car if you parked the car in a very cold environment.
In any case, regulating the battery temperature consumes power itself (running the liquid pumps, cooling fans, heating elements, and/or refrigeration compressor). Because the car doesn't want to drain its own energy by, possibly, needing regulate the battery temperature "indefinitely", it asks to be plugged in. For this reason, the manual recommends that you keep the car plugged in whenever you are not using it.
If you live in a moderate climate, you can get away with not leaving the car plugged in under normal circumstances. But, you live an extreme environment, more often than not you may need to plug in (and the car will let you know).