If you have doubts walk away and forget about it, I'm not a fan of any lock with external access and feel if they are hidden on the inside they don't know what they are messing with and tend to be a bit more reluctant to have a go..
If you're looking for extra garage security I can say my experience with the "enfield garage door bolts" are really good even if you can't get them close enough to throw their bolts into brick they are still quite good when using the bolts to arse (but) up against the doors frame. (
bloody anti Americanism)
I have a metal frame around my garage door so mine poke through the side of the door and behind the metal frame which prevents the door from being opened without cutting away the heads of the coach bolts or sections of the door/frame.