A possible hidden agenda....... Like confused commented/joked about if they were to ever build houses/property on it..... Well who's saying that is not true? No harm in being aware that something in the future could happen. If you or somebody you know lives on the doorstep then you are bound to be suspicious.
Whats all that got to do with someone putting a padlock on a gate to stop people entering.
The most probable and logical answer is that the fellow is locking the gate because the local authority has stopped doing it due to cut backs and all that old bollox-this person may be protecting his peace of mind from feral youths who may be entering this gate at late hour and disturbing his peace.
If this person has locked the gate without due authority then any citizen can unlock it.
If thy are going to build house on it all the above is irrelevant because the authorities would leave the gate open so it caused problems for the local residents.
If you try to cure evil with evil-you will add more pain to your fate.