A bit of a yarn

While the world slept and everything was in almost total darkness, with only the faint light of the stars to guide them, they seized their opportunity.  The only sound as they approached was their low chant of 'knit one, purl one', repeated over and over again, and just the faint click of their needles.  No post box was safe while they were around, and it didn't take them long.  As soon as they saw it, they pounced, surprising it from behind.  The box didn't stand a chance; in seconds its top was covered with a hat decorated with hearts in the colours of the Ukrainian flag and then, their mission fulfilled, they slunk away into the night.  Yes, the Yarn Bombers of Old Holt town have struck again.  Where will they strike next?  Is any post box safe?  We should be told; watch this space for further news.