[Pieter H]

There are indeed persistent reports of a Bf109 shot down into one of the waterways around Rotterdam in the early morning of May 10. The Oude Maas at Spijkenisse is often mentioned.
Some sources state the aircraft to be from 1./JG1.
However, firstly there is no proof from the NVM or other German sources of an identified Bf109 in this theatre. See e.g. the list of Peter Cornwell.
Secondly, I./JG1 as part of Fliegerkorps VIII, was active over Maastricht. I don't exclude that only for the early operations of 10 May over Rotterdam 1./JG1 was detached, but I haven't seen any proof of that. In fact I think it is quite unlikely, since Maastricht was as least as important as the operation over western Holland.

So that's where we are: no confirmed wreck, no confirmed loss of an identified pilot, and the only reference to a unit probably wrong.
Still I think there is a reasonable probability, based on eye witnesses etc, that an aircraft was shot down. But not necessarily a Bf109!

Regards, Pieter