Could be that the person making the report felt the flag infringed Article 5 (3)(a) of the ANO:
(3) Subject to paragraph (4), an aircraft shall not bear any marks which purport to indicate—
(a) that the aircraft is registered in a country in which it is not in fact registered;
If so, then I suggest the individual making the report needs to get a life
How a national flag could make anyone think that a aircraft clearly bearing prominent G registration markings (I assume it was) could be registered anywhere else is beyond me
Aiglon