Lexus IS300h (2013) - Not Unlocking

Forgive me for this post, I'm a Lexus novice.

A friend of mine has left the country for a few months and has left his Lexus on my drive. It's officially off the road and I've been turning it on about once a month so that it's not left idle for too long. However, this time when I went to unlock it nothing happened. The key still seems to be working as the red light flashes whenever I press any buttons. My guess is that the battery in the car has run flat but I'm a little worried about using the physical key to open it as I'm not sure whether this will set the alarm off.

My question is, if I set the alarm off by using the physical key, how do I turn it off as quickly as possible? I don't want to be annoying the neighbours for too long.