Yes, nanners can and do form anywhere on the plant. I even had a nanner mid-internode before on my Shishkaberry.

If it's a seed or a calyx, there will be a couple of hairs poking out of it's pointy end.
A seeded calyx might or might not have a 'racing stripe', but the pistils (hairs) will be dying and the calyx will be swelling.
If it is a seed, where did the pollen come from? (do you know what caused the stress?)
If it is a nanner, get it while it's young with tweezers. Letting it mature till opening will result in a massive loss in yield, and shitloads of seeds.