A proto-warp signature was a detectable emission produced by proto-warp driven starships.
Between late 2383 and early 2384, the USS Dauntless detected three proto-warp signatures. Upon learning about the third signature, Vice Admiral Kathryn Janeway commented that "two times is a coincidence, three is a pattern," before ordering the Dauntless to the position of the last USS Protostar signature at maximum warp. (PRO: "A Moral Star, Part 2")