[Izkata]

This was explained: The secret-keeper died, so everyone who knew the secret became the new secret-keepers. They brought the outsider with them accidentally, which was enough for the magic to be satisfied that the secret-keeper showed them the secret.

[wizzwizz4]

The other time it didn't (revealed in the third book) should be enough for them to at least have their guards up.

Though, given the events of the fourth book, every participant in the war must be incompetent at warcraft – which we do see in subsequent books. You could argue it's consistent with the characters being incredibly complex (traumatised veterans, ego-fuelled tunnel-vision, not actually caring about victory, etc)… but realistically they were written by a millionaire[1] with no experience of war, and are fairly one-dimensional characters.

[1]: Sunday Mirror, Jul 9, 2000.