I hadn't heard of these until I tuned into BBC C&W yesterday. The police are asking to report any suspicious markings on pavements and roads: http://www.bbc.co.uk/news/uk-england-coventry-warwickshire-33799231

I honestly think it's bunkum Lex.

These days with mobile phones and picture taking facilities it's hardly necessary. And why once it's 'common knowledge' keep on doing it?

I've seen many of those signs before, mostly for street utility works.

Whilst this doesn't debunk it, it makes interesting reading...

Snopes