Dark Willow wouldn't try to hide though. She would probably come right after them, or go and try to destroy the world again. She wouldn't be avoiding Buffy and Xander the way Willow did with the spell. Hell, the plane probably would have been full of dead people if Dark Willow was back.
Dark Willow was not serial killer on rampage. She was same Willow as normally, just without (or greatly reduced) moral restraints. Main reason why Willow was suspected was the fact the victim was also skinned alive, something they at that point had seen only once.
Willow did not go to Anya before trying to reach Buffy and Xander, she went after she failed to find them. For her Anya opinion of her probably was just a fraction of importance compared to Buffy and Xander opinion.