Slaughter reports are random sort of.
Usually it happens when your waaay outnumbered. In theory they can happen after any battle but the more lopsided the number of Soldiers (Not units) the more likely it is that your squads will be slaughtered and you not even get a battle report.
This was put in so that you couldn't just send 1 single spam guy and find out every unit at a location every time. By making it less dependable it does reduce it's effectiveness.... Somewhat.
Personally, I'd like to see it at a set ratio. 5:1 or something. Because if you're attacking someone with an army 20% or smaller, than the size of the defending forces, You're not trying to win, you just want a battle report. *Or you suck and didn't think people move units.* lol