https://en.wikipedia.org/wiki/Pioneer_anomaly

It led to all sorts of wild speculation- New physics, Modified Newtonian Dynamics.... in the end it had a much more prosaic cause...

Thermal propulsion via the waste heat from the RTG power source...

Given we do not know if it was outgassing, and can only estimate an uppler limit to how much it could have outgassed, and given that we don't have terribly firm estimates on its dimensions and albedo it seems like any model of how it 'should have' accelerated in its orbit would have to include fairly huge error bars given the uncertainties in solar and thermal pressure...

I would love nothing more than to witness evidence of ETI in my lifetime... but jumping to wild conclusions with lack of evidence is only an insult to the intelligence right here on Earth.