I don't know if that really is what caused the butterscotchyness or not. I hope so, because I've had to track down infections this last year and a bit and man, does it make you consider quitting brewing...
Funny you should mention that; I just replied in another thread
about a recurring diacetyl problem I once had that I attributed to infection. It was every bit as demoralizing for me. On the plus side, I put a lot of controls in place that improved my brewing in other ways.
Also, and I think this is what you might have been getting at, I believe a precursor to diacetyl, acetolactate, is odorless and flavorless. It oxidizes to diacetyl. So you can have the potential for diacetyl in your beer and not know it. Then when you rack it and introduce a little oxygen, suddenly you have diacetyl. You can test for acetolactate by heating up a sample in the microwave and then looking for butter.