Not sure if it's the help you require but found this on B/ham airport car parks site............might be worth a phone call to enquire if inside, outside, secure etc.
Motorcycles
If you’re arriving by motorcycle, we have some motorcycle spaces available, located to the left of the entrance to Car Park 3. These spaces are available on a ‘first come, first served’ basis, so it’s well worth booking ahead online. If the motorcycle spaces are full, please press for assistance at the entry lane, exit lane or one of the Pay on Foot machines located within the car park and our friendly Control Team will be only too happy to help.
Just re-read your post & I think you are asking about any airport rather than Birmingham in particular.