Uncashed checks and cash? That sounds like an underling who never came back for the booty. Osteen wouldn’t leave uncashed checks and cash around in his own church, that’s dead money in a dangerous place. He has much better ways to skim if he wants to skim.
Possibly the underling sealed the money and checks in the wall with the intention of retrieving it later, but then died before he could get to it.
Give the plumber credit for doing the right thing. He could have helped himself to some of the cash, then sealed the wall up again.