In comments on previous exercises, I got the impression that 'rencontrer' was used for chance meetings whereas 'réunir' was more appropriate for planned encounters, which these clearly were.
"Tous les jours ils se sont rencontres en secret" why wrong?
It is correct and accepted.
Pourquoi pas "retrouver"?
I would guess because it was a habitual/ ongoing activity in the past rather than one time and done.?
Tous les jours ils se rencontrent en secret...is it wrong?
Yes, because you need a past tense: ils se rencontraient / ils se sont rencontrés.