Slitherine

There's a fragmented foot spear unit in the woods which I don't think I'm meant to be able to see
No unit info is available on the tooltip, just the banner is visible

Actually you are. The unit formation is messed up by being fragmented, so that one man is actually in the adjacent (non-forest) square. And the game allows you to see him, and then because he is the only one you can see, places the banner there.