The WB didn’t register alarm over an incident witnessed. They offered a fact pattern of a conspiracy and provided a de facto roadmap for investigating it.
1/
Someone with experience in the DOJ, a prosecutor’s office, or law enforcement maybe.
We can rule out law enforcement (eg the FBI) probably.
That leaves someone with time in the DOJ as the most likely answer IMHO.
2/
Coulda been there, been told, or had access through an investigation where they were relevant.
3/
William Barr shuttered an array of ongoing investigations inside the DOJ after he closed down Mueler’s office.
One of those investigations may not have gone to the grave so easily though.
4/
The only place left is Congress.
5/
A member of Mueller’s team came from and returned to EDNY, if I recall correctly.
6/
I’m guessing this whistleblower went to Congress as much to go around Barr as to nail Trump.