BCM? Short? who knows?

So i go to turn my car on after a couple hours, i turn the key and nothing happens. Engine refuses to crank, and all electrical systems flick on for a second then shut off. tried boosting the battery, but it still didnt work, wouldnt even hold the charge. also, when i dont have the key in the ignition, but battery is charging, it sounds like the BCM is clicking.

Check the battery connections.

Could a loose/bad connection drain the battery in 2 hours?

No, but a side terminal battery can go bad at any time. The side posts can deteriorate inside the battery. Once you try to remove the cable the battery post can come out with it. Either that or the terminals are loose or badly corroded. Either way the connection need to be checked before anything else.