They likely have started as they fall away from the pylon. The Stormshadow, and other cruise missiles, use jet propulsion and don’t usually have a very visible exhaust. The reason ground and ship launched versions have the smoke trail is because they use a solid fuel booster to push them up to speed initially for the jet to start. Air launched missiles already have speed and altitude, so you don’t need a booster.

Right? They don’t feel “launched” so much as “dropped”.