What did you see? Cave swallows or Giant Cave Swallows? The latter are "vermin", and thus not real creatures. Also, your uncertainty means it's reasonable to ask whether the webs you see are Cave Spider webs or GIANT Cave Spider webs. The former are created by "vermin" creatures that are common in the two upper caverns, while the latter are created by the feared/loved creature.
Cave Swallows are often found in Cave Spider webs, presumably killed by the web creators. It can also be noted that cave spider webs are just as good for clothing as (and mood satisfaction) as Giant Cave Spider webs, only less valuable.
The creature group mechanics are the same on the surface as in the caverns. This means that a group (for some creatures the size is always 1) appears on the edge of the map, they move around for a while, and then leave via an edge. Shortly afterwards a new group of creatures (of the same or another kind) is spawned by the edge (again, the same or a different one), they move around for a while, and then leave. Your troll is a "group" of one, so if your embark is not (partially) in a savage biome you shouldn't get another "group" in that area (cavern in this case, but the surface logic is the same) at the same time.
Finding dead bodies of non vermin creatures means they have either been killed by some other creature in the cavern, or they have fallen while climbing and died from the fall. In the first case the killer may or may not have moved on. Note that all of this has gone on throughout the existence of your fortress, regardless of whether you've breached the cavern or not.
FBs never leave, but tend to (try to) kill everything entering that cavern. Occasionally a cavern critter from a "group" manage to kill off an FB (and usually leave the cavern), leading to the bodies of its victims as well as itself littering the floor of the cavern.
"Groups" of creatures can get stuck, typically in trees, in water, or just stay motionless when one of its party members has been caught by the player. These groups are called "campers", and block the spawning on new groups because the refuse to move out.
Ah, I see the lecture time is out. Time for a break!