That still doesn't answer it. Even without some Dr. Claw style shadow (side note, one of the worst plot points in any Trek series ever) telling the Enterprise crew who sent it, why would anyone risk being discovered? After all, their later probes are used in faraway uninhabited systems, so it makes no sense to test an even less powerful one to just kill a few million and potentially alert them to either your presence or put them on high defense for any further attacks.
It alerted them they were attacked, with a corpse and ship. A million different ways could have been written for Earth to find out who did it, which begs the question on why even do it to begin with. Also, they bizarrely (or I guess not so much since this is a time travel plot) establish Xindi traveling back in time to Earth trying to build some disease, so why didn't they just send a bunch of powerful weapons back in time and destroy Earth? Why not send the probe to a time period long before to destroy Earth? Time travel plots really only work when you establish you can't just time travel anywhere at will, or else it just opens up a bunch of plotholes. The Xindi plot is just nonsensical when you think of it, and not terribly interesting.The probe did not alert Earth to anything or anyone.