Those leaves are the cotyledons, they come out of the seed which is why they look different. And yes, they're sycamore and my garden is full of them this time of year. Those and ash. They swamp my oak tree unless I pluck 'em (carefully nurtured from an acorn, I always wanted an oak tree). Quite a few had already rooted in my garden and they're buggers to get out (gardener mostly just cuts back the new growth).