To answer the question seriously, I believe it's the TNG tech manual which says that photon torpedos and such have warp sustainer engines...essentially a torpedo/probe fired at warp tends to remain at warp.
Well, for a little while until the sustainer engine runs out of fuel and then it drops back to sublight (but the DS9 Tech Manual
says that if a photon torpedo misses its intended target and flies out of range, it will self-destruct).
Back to the TNG Manual
, though, it does list that Class-8 probes are capable of accelerating to Warp 9, but it can only maintain that for 6.5 hours, which roughly matches the little over six hours the Enterprise
had to rendezvous with K'Ehleyr's probe after it left Starbase 153.
It could be argued that there are such things as mini-warp engines, but due to their small size, they can only maintain warp for an correspondingly short period of time.