I disagree with you, but it would probably be an impossibility for me to prove it! Why? Well, as soon as a truly altruistic act is observed by third-parties, there's a chance the actor could be rewarded for it (for instance, by being honoured by society). Then, we would have at least the ability to claim that the actor was doing it for the chance of a reward, rather than purely selflessly. Thus, truly altruistic acts can only be immune from accusations of selfishness if they are entirely unobservable!

Of course, I hope you'd agree that such an accusation would not always be plausible - the higher the personal risk of the act relative to the potential reward, the less plausible selfishness is and the more plausible altruism is.

I personally have acted altruistically - as you describe it, "actively and knowingly doing good for someone else for absolutely no personal benefit". I can't prove it to you, but I think it's a natural consequence of empathy. I'd posit that altruistic acts are often done by those in situations where they are vulnerable themselves, which is why events like natural disasters are often accompanied by unusual levels of social cohesion and acts of personal sacrifice. That instinct can be exploited, of course; yet, by itself, it is a beautiful thing.