Having missed his best chance to kill Peter Pettigrew, Sirius was nearly driven mad with rage.
Now, with Voldemort's help, Peter Pettigrew had successfully turned into a rat and slipped into the sewers, vanishing without a trace. No matter how furious Sirius was, it was useless now.
Felix Felicis told him that after making such a huge fuss, only to fail to corner Peter Pettigrew, the Netherlands Ministry of Magic might very well take its frustration out on him.
Sirius had considered staying to continue searching for Peter Pettigrew, but he quickly gave up on the idea because it was pointless.
After escaping into the sewers, Peter Pettigrew certainly would not remain in the country. Who knew where he would hide?
Besides, Harry's Second Task was about to begin. Although Sirius had asked Remus to look after Harry before leaving Britain, he still felt he could only rest easy by returning to keep watch himself.
In the end, urged on by Felix Felicis, Sirius examined Voldemort's infant corpse. After taking several photographs of the ugly, snake-faced baby, he hurriedly left.
As for Voldemort's infant corpse, Sirius did not pack it up and take it with him. He had to leave something behind for the Netherlands Ministry of Magic, or they really would have gone through all that trouble for nothing.
After returning to Britain, Sirius immediately contacted Albert through the Two-Way Mirror, informing him that the attempt to ambush Peter had failed. While he was at it, he asked whether Albert had any reliable channels for buying safe Felix Felicis.
As for buying Felix Felicis on the black market, forget it. Who knew whether drinking something bought there would poison him?
"It failed."
Albert was genuinely a little surprised when he learned about it.
Sirius had made ample preparations before