but then waving your hand 3 feet away does the same when the nothing is plugged in.
This is the simplest way to test whether the electrometer input is still alive.
The dielectric noise will be lower than the noise of your amplifier. Therefore, it is not noticeable.
P.S.
Unfortunately, I am not able to test this amplifier.
Its parameters and design look very balanced.
But I don't believe in 1E-15A noise free, only if with a very "evil" filter, with a long settling time.
I think you used a 10GΩ feedback resistor for the low range?