I planted a little corn this year and a bunch of corn came up in places I didn't plant it. I originally thought this was my son getting a hold of a seed packet and liberating its contents, but further investigation has me doubtful.
I pulled a small seeding and looked at its roots. I noticed the seed it germinated from was about 1 mm across, the bigger versions are starting to develop branches at ground level, and the base of my corn is turning red while the base of my 'corn' is not.
My quick internet searches have shown me to sorghum and johnson grass, but I can't really tell from pics I've seen.
Looks like the sorghum I planted (on purpose). Sorghum is also known as "birdseed", so all it takes is a spill from a bird feeder and you have yourself a nice stand of sorghum. It's too thick around the base to be Johnson grass, I also have that. Even at their widest, Johnson grass leaves are rarely more than 1" across, while sorghum will be as broad as corn. In fact, it's kind of hard to tell my sorghum from my neighbor's corn at this stage of growth.
If the sorghum looks nice, let it grow. You can harvest it later for (a) more birdseed or (b) making jowar flour or (c) extracting the juice from the stalks and cooking it down to make sorghum syrup.