JanM, on 2014-April-06, 22:51, said:
It's been suggested to me that if 3NT is natural but can be made with a hand as strong as this one, it is probably an alert. If that's correct, there wasn't any UI from the alert (and from the OP I think that no explanation was sought or given until after the auction was over). In the absence of UI, both players bid somewhat reasonably in their own "worlds." North thought that 3NT and 4NT were RKCB and responded to both of them properly. S/he didn't pass 5♦ because S had asked for Kings, which is surely forcing to slam. S/he bid 6♦ to show a minimum with no interest in bidding more. South thought that 4NT was RKCB for hearts and that 5♦ was to play if partner had 0 Keycards. S/he passed 6!d because at that point s/he knew that partner was apparently on a different wavelength.
I asked South why they had bid 3NT, and they realised it was not a good call, but considered it preferable to making a forcing bid in a major (promising 4+ cards) and in clubs (promising 5+). They didn't have a forcing diamond raise available. I went to some lengths to find out whether there might have been another alertable meaning for 3NT in their system (e.g. a natural bid with a hand very limited in strength and shape), but I couldn't find any.
I think NS bid reasonably in their own worlds, but the law requires more than that when they're in receipt of unauthorized information.