Well, this is a weird one. Client deleted an unused bank code a while back, and today the prenote wouldn't generate because of a Missing Prenote Bank Code. The bank code selected when creating the prenote was still out there. After much mucking about, it appears to have something to do with the PR_005 file, (which isn't in the TRSG), which had a record for the deleted bank code but not the current one. Looks like the prenote record in PR_03 doesn't grab any actual bank info (routing, bank account) when creating the record, so it had apparently been using the ""1"" for quite some time.
Thought about contacting Sage, but then decided it was too much effort. Sads. How many other bugs exist that aren't mentioned because it's too much effort for too little reward? The work around is just fine, etc, etc.