It's not uncommon. Every year there are training camp retirees, typically older guys around who maybe want to give it one last year but decide their body just doesn't have it for another year.
Believe me, the Saints weren't relying on contributions from P Rob. They've been vocal about a need at corner and that was with him on the roster. If he trotted out at #2 corner to open this season, something would have went very wrong.