Maybe they just forgot to indicate it and can't quite remember during the post analysis which one they moved?

I guess its possible that they didn't realize that another piece could have moved there, which may indicate that they aren't seeing the whole board. But, with new players, I think just going through a game and seeing the blatant mistakes (like hanging a piece) and all would be a better indication.

I encourage newer players to use the long form algebraic notation, "Ng1-f3," until they are familiar enough to use the abridged version. 

I believe it's the case that if you seal a move this way in an adjournment, the opponent chooses which rook.

No. If the sealed move is ambiguous, then the player who sealed the move loses the game by forfeit. FIDE Rule 8-a under "Guidelines in case a game needs to be adjourned".

http://www.fide.com/component/handbook/?id=125&view=article

OK, that's even stricter than I thought.

Adjournment is a rare occasion these days - there was one event last year which allowed it.  The advent of computers has lead to the general imposition of an idea originally Fischer's (at least he was the first top GM to advocate it), that games be played to a conclusion in a single session.