Wow! Could it be that plant is a tomatillo? Strange that seed would be in a packet of tomatoes. Let us know what you find out. You have me curious. :)
Tomatillo was EXACTLY my first thought.
Tomatillo seeds do look very much like tomato seeds, so the possibility of one getting into the seed packet of the other doesn’t seem so strange.
If it is a tomatillo -— I am waaaaaaaay jealous because none of mine ever came up, and I love tomatilloes.