Daft question time, what indicator do you mean? I checked my car after I posted, and noticed my front indicators are an all-in-one unit with the headlights, there's nothing to fall out?
Or do you mean the side repeaters? If so, I would use some good silicon sealant. It's tacky enough to use as an adhesive, obviously waterproof, just put some on the back, push the indicator back into place, wipe off the excess and use masking tape to hold it in place until it's dry. At least then you'll be able to take it back out again to change the bulb if necessary.