Proto-warp signature
A proto-warp signature was a detectable emission produced by proto-warp driven starships. 

Between late 2383 and early 2384, the  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  signature at maximum warp. ()

Warp