tigger:
ignore the advice to look in "Other devices" in Win 10 to see if driver installed, it's actually in Ports(COM and LPT)
It was there because the driver was already installed. If no driver was installed it would have been in "Other Devices".
tigger:
All I can think is that a careless flick of the cursor changed it.
More likely what happened is the Arduino IDE showed a notification that an update was available for your boards, then you installed Arduino IDE 1.6.21, which is the first version to have Tools > Processor > ATmega328P (Old bootloader). Default selection is Tools > Processor > ATmega328P.