There's not much else it can be if hotspot shows something. Its pretty conclusive. At least on the outside edge, inside could be bat clipping the pad or whatever.The interesting thing about hotspot is we've seen players given out simply based on a white mark, nothing else. There was a dismissal earlier in this Test match (can't recall who) where all the umpire saw was hotspot and bang, out.
Not showing anything could be minor contact, or a complete miss.