Indeed the red light indicates doors are going to retract. It's positioned on the keel beam. I'm aware of this on the 777 and 757, so it's certainly a Boeing thing. Don't know about Airbus. Pax aircraft do also have regular lights in the wheel well. On most Boeing aircraft you have swithes to put the lights on/off (757 on nose gear leg and behind main gear doors). The A300 has the lights in the main gear compt always on on ground. There are even cases where the aircraft is grounded when these ordinary lights are not working. Because it's necessary for the pilot to do it's preflight. Had this once, but since all our pilots have torches they leave it as is now.