He was at the villains base where he stayed with the League of Villain's he was a part of. He was simply in the bar of the base. The base itself wasn't an abandoned building, and it's where Dabi called home.

Everything was all good and fine, that was until the villains, who were all in the bar, heard a knock on the door. There was confusion all around. But one of the other villains, Toga, went to the door to see who it was.