An owl was sent ahead, warning that Bill was on his way, and so Alastor was ready. He didn't get many guests, so if somebody were to show up on his doorstep, unannounced, Alastor wouldn't take it so nicely. This was even more the case with the Death Eaters in charge of the Ministry.
His home was small, utilitarian, and warded with so many protection charms, it would make your head swim. As an Order member, Bill had prior permission to get through the Fidelius Charm, otherwise, it wouldn't have even visible. Alastor saw him coming, viewing Bill through the walls of his home with his magical eye, so when Bill came on the porch, Alastor was there, opening the door before there was a chance to knock.
"Get in here," he said, gruffly, not wanting to keep the door open for long. Once the door was closed and re-locked, Alastor was found to be holding his wand out. "This ain't no social visit. What's the matter?"