Tenants swapted an old boiler for the good one in the house?

Ok i have a bit of a situation. My fathers tenants were complaining that their boiler was leaking so my dad went to fix it and he realized that it wasn't the boiler that used to be there (that was practically brand new). We have video AND pictures of the the original boiler from when we were trying to rent the house out. We're a bit dumbfounded that they thought they could get away with it and now we don't know what to do next. These tenants are supposed to leave at the end of next month and my dad thinks the boiler is hidden in the garage. What do we do. We know for a fact its not the same boiler.