>
> I suspect unused pins should be left as inputs, isn't that why
> they are reset to inputs in the first place?


Not exactly.  It lets you use pulling resistors to set the output pins into
safe states without having to fight the chip outputs.



> And why would touching an input would have any effect on the pic,
> unless there is some firmware which is monitoring these pins, which
> there isn't, I can't see why this could possibly be the reason, and
> maybe it is a capacitance issue,  i.e normal  expected behaviour?


An open pin can float to VCC/2 where the input circuitry will draw some fair
current.  They can cause undesired interrupts, and interfere with other
stages on the chips.
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