It was in series with an 18M2 (Brown, Grey, Red, Green) resistor across a Vishay S102L precision resistor (20K000 0.05%). This resistor is part of a precision attenuator, and the R and C were mounted under the board. The other resistor in the attenuator (289.3 Ohms) also has series RC trim under the board in parallel with it (274K in series with a cap marked n22 AFAICT, so maybe 220pF).
The two main resistors appear in the schematic, but these under board parts don't.
If I had to guess, I would think they are there to trim the frequency response of the attenuator.
RU sure that it's 1pF? Finding a leaded 1pF cap will be non-trivial.
Dave