Strange warm up behaviour

For the last couple of weeks I've been noticing a strange warm-up behaviour on my 2010.

This morning I went on a short trip (8km total) to run some errands. I drove a few km then stopped at once place, and left it in ready mode as I was next to the car. The ICE was off the whole time, so I assume the car had reached S4 mode. After that I drove a couple more km to a fuel station. I filled up (prices have gone down by 10c!), checked tire pressures, then drove off.

There are traffic lights just after the exit from the fuel station, so the ICE didn't start to run from when I switched on the engine until they turned green. It's a quiet Saturday, so I took off slowly, staying at the high end of the eco zone. The whole time I was accelerating (to 50km/h) the ICE stayed in warmup mode, the fuel consumption hovering around 7l/100km (33 mpg). During this time the battery dropped 3 bars, so it seems like only the battery was being used while I was accelerating. I believe this has happened a few times over the past couple of weeks, as at times I've noticed the battery unusually low (2-3 bars) where it would usually be higher.

I should note it's been unusually hot here the past week (30c+ every day), so I've had AC running all the time. The power mode was normal (no lights) and I wasn't using EV mode.

Is this expected behaviour, because I've never seen this before (I've had the car 3 years)? I'm wondering if it's going to be more efficient (or at least cause less wear and tear) to force the ICE to be used by accelerating harder, or at least pre-emptively starting the warmup cycle by pressing hard on the accelerator at the start of the trip.