Lower exhaust pipe leak discovered after transom/outdrive rebuild

I had my transom and outdrive serviced on my Mercruiser 3L 140/Alpha One and when the boat was launched the mechanic discovered that the lower exhaust pipe was leaking and the boat was taking on water. He said there was no way for him to know that the pipe was leaking BEFORE he launched it. Now, he says the engine has to be removed in order to replace the pipe. I don't doubt his honesty nor his expertise but my lack of experience in marine engine work makes me question the need to remove the engine to do the necessary repair. Am I being overly cynical here? Is there really no way to anticipate a leaky lower exhaust pipe prior to launch?

Is there really no way to anticipate a leaky lower exhaust pipe prior to launch? Nope
you should have noticed the boat taking on water last year;

Now, he says the engine has to be removed in order to replace the pipe
unless you can access the 4 bolts here, the motor must re removed to replace the pipe

Thanks Doc for the quick reply. You've eased my doubts about this issue and now feel a lot better about it. Great forum here. Glad I found you all.

Doc,
I just realized something I hadn't mentioned to you before. The, mechanic was supposed to run the engine before doing any work to the rest of the drivetrain. When the engine is tested out of the water don't you have to provide running water (a hose) in order to run it? At that time if there is a hole in the exhaust pipe that would be fairly obvious wouldn't it?

No ,not really. I did a job where I had wire brushed and cleaned a Y pipe and nothing showed on land. When launched, the backpressure from the water in the exaust pipe blew out a piece of the corrosion and it started leaking.

Doc, I certainly understand what you are saying and it sounds possible that could be the case. However, if the hole was already there (not just a thin wall waiting to blow out) then my contention is plausible, no?

the exhaust downpipe is not attached to the engine when one runs it on land. you just let the exhaust blow out the riser. (you can probably watch a bunch running on youtube to see what that means)

There are a few things that can leak and cause water to run in and off that exhaust pipe. Unless you have your eyeball on a stick, or can feel the hole in the pipe with a finger, there's no way to really tell it's the pipe or something near the pipe until the engine is out. If I were you, I would try to be there when the engine comes back out. What I would do is pull the motor, relaunch the boat and lay in the bilge on my belly or hang in there upside down while the boat floats. That's the best way to be sure where it's coming in. Hydraulic manifold is right next to the exhaust. It could be that or about 6 other things.