I bet this some output from simulation software. This result is just an error caused by imprecise calculations. I think you could never measure that small current in reality, especially on high-voltage three phase line.
Hypotethicaly:
If you imagine a piece of wire connected on one side to Earth (like in your case), anything insulated connected to the other side (chassis, etc.) will create a small capacitor in respect to Earth. My guess is that current induced in this LC tank by all the radio waves and static will be orders of magnitude higher than 10pA. So some current through the "disconnected" wire is possible, but highly irrelevant.
Anyway, because the wire you are talking about is neutral, this also can be casued by some very very very small imbalance in the load, which in reality will be also at least 1'000'000'000x higher.
In a perfectly balanced three phase system, there would be no current flowing through the neutral conductor. The impedances in such three phase system are balanced when they are identical in magnitude and angle. Based on the 10 pico Amps readings (or calculated values) you mentioned, it can be said that such system is very close to be a balanced three phase system. On the other hand, if the impedances of a given three phase system are not balance, it is then when the sytem will be experiencing a current flow in the neutral conductor. Mathematically, this can be verified by using one of many methods such as vectors analysis.