Hi, Chris.
It is possible that your grandfather was on board a He 115 that was slightly damaged by Flak in the Sognefjord in Norway on April 15th 1940.
A Leutnant zur See Müller was wounded in the incident, the aircraft from 2./506 so slightly damaged that it was not reported (well in fact first reported as damaged and then stricken, in Luftwaffe terminology this would indicate a damage of below 10% for the airframe).
Please see the following link for the loss report:
http://www.ahs.no/ref_db/lw_loss_pub...?lossid=116136
It is possibl that the Oberleutnant Heydt mentioned is Hans Heydt that was later reported MIA over Scotland:
http://www.ahs.no/ref_db/lw_loss_pub...?lossid=113836
It is possible that there are others on the board that can add to the last case.
Regards,
Andreas B