I don't think this has anything to do with IR signals.

If it is related to IR signals, I don't think it would be "stray IR" like you're thinking. It would be more like: Something wrong the circuitry causes the IR sensor to behave as though it's getting stray signals even when there are none.

Question about the Mk2a volume control: It's optical, right? Is it infrared? Maybe the casing of the encoder leaks a bit...