They have put one on every property....
They contract out the erection of signs and the workmen would have had clear instructions on which properties to put them. They have clearly been told to do the whole street. My house number is on my gate and my property and my fence would not be mistakenly thought to belong to another property. It is very distinctive.

I hate this new trend of estate agents boards littering the streets. They normally keep them up weeks after the event providing free advertising and looking ugly.