Just got an anonymous letter - WWYD?

I opened the door this morning to find a sheet of paper under a stone, letters cut out of newspaper, saying "you hit an ol man on xxxxx rd we don't like bulllies" It shook me a bit and I'm not really sure what to do. Clearly we haven't hit any old men so we can only assume its kids, or someone with the wrong address. We get school kids coming past our house and get the occasional "knock and run" and the use of the word "bullies" makes me think it probably is kids, but it's made me stupidly nervy! We are moving shortly so I'm inclined just to ignore it but is all a bit odd!