Oh, I know! I was already a wibbling mess by that point (that's what happens when you mainline Season 4 in one day, sitting on your sofa and clutching your cat for comfort), so mostly my reaction was one of incoherent screaming at my TV set by then. ;)

Since Dean doesn't know about how his message was altered and Sam is never going to bring it up, I agree that accidentally finding out about it is pretty much the only way that could happen. It was either that or have Sam accidentally reveal it during delirium or something. This way felt a little less forced.