The sound is the same as the one you hear when you plug in and unplug a USB device. It beeps when I approach the laptop and leave it and also sometimes when I move in front of it. This is an ASUS, model G73JH.
What you are experiencing is the ASUS SmartLogon feature. It is a facial recognition feature that logs in for you and logs out automatically.
Here is the link to the ASUS SmartLogon user guide: