tree 11 - and a bit of a mystery

Well here’s a turn up for the books. 

I expected little or no change since last month.  And there isn’t.  Not to speak of.  

I also thought that, apart from the weather, it would look more or less like it did the first time I came here, back in February last year.  But it doesn’t. There’s something in this month’s photo that wasn’t there at the beginning.  It’s also there in December and the eagle-eyed might even make it out in November's blip.  I'm not sure.  

To the left of the trunk and deep within the canopy is the remains of some sort of shelter.  It’s certainly not there in February or even April.  It’s impossible to tell over the summer when it first appeared.  

So - what is?  When was it built?  And why did I only just notice it today?