When I pick a pod it's normally not cracked open by itself yet, just looking or feeling very "ripe" to me. Because of that, once or twice I've gotten no-seed pods. I've thought that they must be pods that I've just happened to pick at the moment right before the they would have begun shriveling up (and I agree, those can get to a good size first).
I remember seeing a few tiny "seed shards" like you sometimes see in other pods with one or a few seeds, as if the pod had been in production and the seeds had just not gotten past the very earliest moments of development. I'll try to watch for them this year, and see if my camera can take enough of a close-up to show what I mean ...
