A tree appeared out of nowhere!

I know we are now into the growing season, but I was surprised to find a new tree in the garden which seemed to appear overnight.  I am sure in reality it must have been growing unnoticed for a while but perhaps because it didn't have leaves before and I've not spent much time in the garden recently I didn't notice it, or can they really grow that fast?!  It's in the untidy corner of the garden (lets be honest, all the corners and sides are untidy) where the trampoline was last year so it definitely wasn't there last year, and it must be 6ft tall already. It's a sycamore, not from seed but growing from a root from a bigger tree next door.   I shall leave it to grow as it's made such a valiant effort but won't let it get too big as I already have another sycamore that needs regular haircuts to stop it getting massive.