I've noticed mine seems to take the opportunity in hybrid mode to start and warm up when I'm slowing down. The RPMs are a little higher. It doesn't always do this. But seems to be every day or two if I've only used electric. It doesn't do it in electric mode.
My guess is that it is taking an opportunity to warm the engine/flow oil under no load.
Under normal hybrid operation, I often don't notice the engine unless at idle. Which it doesn't do very often, except as mentioned above. You think, what's that strange noise? Oh the engine!