Battery drained, cannot open the door but there is a clicking sound?

I drove the car 3 days ago on the highway and then parked it at home. Today, I can't even open the door with the key fob on me. The alarm inside is still flashing, but the battery isn't enough to sense the key fob or open the door. However, I hear a clicking sound coming from under the hood. What's making that noise? Car is not running and I'm not trying to start it. I also noticed that the rear signal lights were blinking. I checked that the hazard button was not depressed and that only the rear lights were blinking. No other lights were on or blinking.