Thanks for your replies. It seems it remains a mystery which is strange for such an important claim which resulted in a significant award.
I understand that records of losses are not complete, but I have a suspicion that this event may not have occurred. There must have been tremendous pressure to provide results to justify the time and money spent on experimental aircraft.

Horst's words:
'In connection with LW-Ministry and Messerschmitt on my request 2 Me-109G have been reconstructed and specialised for higher performance. All unnecessary items with weight and resistance was removed, the engine-power improved especially for high altitudes by GM-1.'

However, I will keep on searching for Horst's Mosquito.