ID please

OK so I used spent compost  for sowing this years toms and should expect the odd rogue seedling. I'm used to weeds and tomatoes popping up everywhere,  but I've got a rogue something coming up alongside some tomato seedlings and it looks strange/familiar/interesting, but I've no idea what.  Its unlikely to be floral considering the compost was old tomato pots.
For size you'll see an emerging tomato, not yet reached proper leaf stage. What is it?........ Inspiration just hit me, could it be a cucumber, although I'm damned as if I can figure how it got into outdoor tomato compost.