RX100 and High ISO

Digital Nigel wrote:

I've been doing some experiments and have discovered behaviour I hadn't noticed before. If I shoot in P mode in very low light in RAW or RAW+JPEG, it just takes a single shot at max allowed ISO, and there's no delay in seeing the review image.

But in Fine mode, it behaves differently: I can hear the shutter firing several times (3?) and then there's a delay of several seconds with a Processing image message, before the review image appears. This has less noise than I would have expected at high ISOs. So, is it just doing some intensive noise reduction, or is it running an undocumented multi-frame mode? I suspect the latter.