So, what - there is corrosion on the PCM? Is that what they are saying?
Did they pull the module and SHOW you what they found?

How long have YOU owned the car? Real water damage to electronics dose not typically take a long time to show up (I have spent 29 years repairing electronic assemblies - I am not stating opinion, I am speaking from experience.) If you have owned it for some time, you'd KNOW if there was flood damage.

Now, I have seen corrosion on electronic boards that were NOT actually water damaged! Often, it's caused by condensation due to very high humidity. When that fan runs that pumps passenger compartment air into the compartment with the PCM and RCM, things should cool down. But then, when you shut the car off, it is POSSIBLE that, due to their low temp from all the air from the passenger compartment, that humid air seeps and and condenses water on the boards. Perhaps THIS is the cause of the condensation they are seeing. I am still not sure if you can get the warranty to cover this, if this is the case.

I'd sure like to isolate just WHERE the breakdown is. I had the same problme with my Ford Ranger once - turned out the nut holding the battery cable on the starter had loosened in the previous 150,000 miles. Tightening it solved the problem. I am unsure of their diagnosis - a little corrsion on a circuit board does NOT nessesarily mean the board is defective. And remember, auto techs are not electronics techs (usually), they are mechanics more than anything.

It SOUNDS to me like they are guessing.

EDIT:::: I sure would like to get under the hood with a DVM and see where we are "loosing" the signal here. I'd still suspect the RCM. Hell, could be a bad RELAY on the RCM. No way to know without substituting the RCM or replacing the individual relay.