[Highland Park] perhaps it's a plague house...

There was another red X marked on my sidewalk this morning. I scrubbed that one off, too. Because I have nothing better to do, I wandered up and down my street - there were several similar Xs marked on a couple different houses in the 900-1100 blocks of N Euclid. Gas company uses blue spray paint, Duq Light uses orange. This is red crayon/chalk. I am posting here, because again it doesn't seem like quite enough to call the police, but I am getting annoyed and creeped out. Does anyone in the neighborhood have any idea at all what this is about? Maybe there's just some adventurous child roaming the neighborhood having fun with grown-ups? Thanks for putting up with my minor obsession, but I'd love an answer.