Stuff like "chases flush draws without odds
" or "chases gutshots without draws", "check-raises strong draws on turn", "pot-sized bet on river = bluff", etc.
Stuff that's not going to be visible in the HUD.
It also pays to replay your opponents' shown down hands in your tracker when reviewing sessions (especially the ones you have a lot of hands on and are likely to face again in the future).
Replaying their hands from their perspective can give you a good insight in how they play, and how to exploit them. It also makes for some very solid notes in terms of bet sizing and the likes.