From the 2016 RC300h handbook
"Switching to EV drive mode when the gasoline engine is cold
If the hybrid system is started while the gasoline engine is cold, the gasoline engine will
start automatically after a short period of time in order to warm up. In this case, you will
become unable to switch to EV drive mode.
After the hybrid system has started and the “READY” indicator has illuminated, press the
EV drive mode switch before the gasoline engine starts to switch to EV drive mode."
Don't know if it's the same for your '15 IS300?
From the 2016 RC300h handbook "Switching to EV drive mode when the gasoline engine is cold If the hybrid system is started while the gasoline engine is cold, the gasoline engine will start automatically after a short period of time in order to warm up. In this case, you will become unable to switch to EV drive mode. After the hybrid system has started and the “READY” indicator has illuminated, press the EV drive mode switch before the gasoline engine starts to switch to EV drive mode." Don't know if it's the same for your '15 IS300?