My guess is that it is Felix Felicis, liquid luck. It’s golden, which matches the description, and it provides a reasonable false explanation for how the invaders were able to break Bella out of prison. It throws you on the trail of a master potion maker, instead of the truth.
But wouldn’t actually using Felix Felicis be even more effective?
It seems probable that Quirrelmort is himself a master potion maker, in addition to the rest of his skill set. He’s able to make the resurrection potion, after all, which is likely a difficult feat.