Makes me wonder what these garages do lol I've come across this a few times at work! Last time it was a bin loader relay causing the fault. Every system runs through a fuse or relay in some way. If you put an amp clamp around the battery negative you can see what load is being pulled from the battery's. If you then say put the lights on your see that load go up! Any thing over 1/2 an amp to an amp is a problem. If you can see a drain on the meter pull each fuse / relay till it go's. Then investigate every thing on that circuit