Are you connected on LinkedIn with your neighbor? Because if your neighbor was connected with him, that could be how it happened. I think it will show if any of

Are you connected on LinkedIn with your neighbor? Because if your neighbor was connected with him, that could be how it happened. I think it will show if any of your connections know him (up to 3 hops away), but you might need to visit his profile to see that info (I don’t use LinkedIn often but noticed that feature before).