In reporting the acquisition of the collection of the late George W. Wray, Jr. (1936-2004) by the Atlanta History Center (URL below), a monograph by Dr. Gordon L. Jones states: "The Wray Collection also includes a shot-torn first national pattern garrison flag flown by A SIGNAL CORPS DETACHMENT AT BATTERY GREGG IN CHARLESTON HARBOR IN 1863 [emphasis supplied - dwg]. This flag was flying over Confederate positions during the July 18, 1863, attack on nearby Battery Wagner by the famous Fifty-fourth Massassachusetts regiment."
I have not encountered another ref to a signal corps detachment or station (often a few men) having national colors -- if needed, a signal station may have used its red (white center) flag as an identifier or location marker. I would suspect, therefore, that this may have been "the garrison flag" of the battery itself. Has this flag come to the attention of others?