How is Dunstan's absence regarded in Silas Marner?
Dunsey's absence is not regarded, actually. He is one of those persons that are better off gone away somewhere where he cannot hurt, rob, or mess with other people. To the people of Raveloe, Duncan's absence was not attended to. They basically felt that he is a happy go lucky guy and he would leave and come back to town as he pleases. Since he is such a crazy spirit, they may have assumed that he went somewhere to do mayhem. The thing is that nobody connected his disappearance with the robbing of the gold.
To Godfrey the story was different. He basically thought he was taken for a fool again by Dunsey and that he took Godfrey's horse, Wildfire, with him instead of selling it like he was supposed to do.
In general, Duncan's reputation was so bad that his comings and goings are expected. However, perhaps for the respect people felt for the Squire, they never wanted to check on whether Duncan was the thief of Silas's gold or not.