[PatrickHenry]

My Hypothesis: the probes have acquired a small but significant positive charge, by having electrons knocked off by cosmic rays and the solar wind.

Ought to be some way to use that charge as an energy source. Even if only to run the transmitter. Or at least to drive a detector to see if it's real.

[Physicist]

Ought to be some way to use that charge as an energy source.

I don't see how. It needs to be connected to a ground so that current can flow out of the probe^*. Just as you need a temperature difference to exploit heat energy, you need a potential difference to exploit an excess of charge. Large potential alone isn't enough.

---

^*Counterintuitive point of science history: current is defined as the direction opposite to the flow of electrons. This is the result of a choice made by the first particle physicist, a founder of my country who is also my employer: Benjamin Franklin.