While searching for something else, I stumbled on this thread and found it worth bumping up.
Yeah, very familiar. In my job for an OEM, testing and calibrating measuring instruments that we manufacture, leakage always is an issue. We're talking about an instrument with a specified input impedance of > 1 TOhm.
Cleanliness of the boards is very important. All sensitive boards are washed in an industrial pcb washer using special detergents. This is to remove the spray on flux used in the wave soldering process. This washer also dries the boards, and surprisingly, not much time between washing and testing is needed at all.
We found the biggest contributing factors to leakage were actually what types of plastics were used in connectors in combination with the weather.
Certain types of plastic (like the green Phoenix connectors) can hold quite an amount of moisture and cause leakage in warm, muggy weather that we usually have here in the summers. The seemingly trivial act of changing to a different type of connector solved a lot of issues we were having during these months.
So, with regard to your meter, just the weather conditions may have an effect on it that's not negligible.
Comment on the video: it looks like a crude way to clean the board, but it's tried and tested, been there done that. That curve on the monitor after cleaning looks very familiar...
Also note that there are no connectors on that board with plastic insulation, just individual pins with air around it. In these low current conditions, plastics turn out not to be that good an insulator at all since it can hold moisture (depending on the type of plastic, as written above).
Also, one of Dave's earlier vids:
sounds pretty familiar. But "my" instruments aren't that sensitive that "someone farting across the other side of the factory"
has a measurable influence...