We have a feeling that thousands of Benedict Cumberbatch fans took to the Internet on Monday to map out Lynn, where their favorite actor was spotted throughout the day. Cumberbatch, who plays Billy Bulger opposite Johnny Depp as Whitey Bulger in the adaptation of the book “Black Mass,” filmed a St. Patrick’s Day parade scene on Mt. Vernon and Union streets Monday morning. The city served as a stand-in for South Boston in the late 1970s. Extras were told to arrive at 5 a.m.