>>774317If it's warm enough, you can whenever you like (see my posts above, seedlings randomly popped up in the garden)
>>774318Soil contaminated with tomato seeds? This happened to me last winter while growing some other stuff (I think it was a watermelon) indoors with garden soil I picked up from outside (didn't sterilise) and 2 tomato seedlings popped up in that same pot in the middle of December (you can see one of them in the lower part of the pic), they're now outside in the garden with the other tomatoes, already flowering
It could be another nightshade in your case though, they often look similar in early stage. One way I could tell it was indeed a tomato quite early was (very gently!) rubbing the leaves and it gave of the typical tomato plant smell
