I guess another way to figure it out is to consider the Z's AI. If you hide and they ignore you and walk past, then they were part of the Wandering Horde. If they run past you and stop where a Screamer screamed, then they were part of her reinforcements. If they run past you and stop at a spot (maybe marked by trash on the ground) in the POI, then they were part of the POI's Sleepers.
I don't think there are any other sources of groups to consider, except if you are in the Wasteland at night then when an out-of-group (Sleeper, Screamer, Wanderer) Z dies, another Z immediately respawns nearby and might run to the location of the last death (if it was a noisy death). In the future when you encounter a big group and it's not the weekly Horde event, then just find somewhere to hide and try to kill them with stealth so that the noise doesn't make things worse.