Another minor mystery

A week ago, there were tens of thousands of people running past here, and leaving however many squillions of water bottles, sweet wrappers and assorted other detritus. A miracle of cleaning-up gets under way almost before the last dogged competitors have staggered past, with most of the rubbish gone before dark the same day; but some odd freelance signage, like this, survives.

At least, I think this was something to do with the marathon - but whether it was a message of encouragement to a runner, or to do with some music promotion, it seems unusually small and discreetly tucked away under the foliage.