After looking in the MTR and IPG, I couldn't find an answer to this. Is a player allowed to wait to point out an opponent's missed trigger until a more advantageous moment.

A reasonable scenario for this is my opponent plays Thought-Knot Seer, and I remove it with the trigger to take a card from my hand on the stack. My opponent says nothing about the leave the battlefield trigger so I let him take a card with the ETB ability, then point out the missed LTB trigger, wanted the card drawn after the previous trigger has resolved.
1. Is it legal to wait until I want my opponent's missed trigger resolving to point it out?
2. In this specific case, is it relevant that the missed trigger (Thought-Knot Seer leaving the battlefield) is detrimental?

Hi Patrick!
I pointed out a couple of similar situations from last RPTQ in Madrid just last week. With this world full of werewolves and triggers, this topic is pretty relevant!

It may look weird, but there is actually no obligation to point out a missed trigger of the opponent as soon as you realize it was missed. On one side, taking advantage of it may be seen as part of the “compensation” for the error, on the other side, a policy avoiding it would be something like “if a player wants to notify a missed trigger of his/her opponent, he/she should do it as soon as he/she realizes it” and would be really hard to enforce!
The only limitation to point it out and take the most advantage out of it is hence the turn time window, and the fact that the ability is detrimental or not is not relevant here: it just changes the penalty ;)

There was a Knowledge Pool Scenario about that:
http://apps.magicjudges.org/forum/topic/21035/