External symptoms of entrapment are the loss of all metadata attached to the avatar of the affected Denizen except for the claim that it exists. As such, it's impossible to tell a trapped Denizen apart from an elaborate Puppet, excepting the orphaned entry in the public Sanctuary Denizen list.
Connection logs will show that the Denizen entering the Fragment they were trapped in, but reveal no log-out, usually symptomatic of a switch to a private Fragment.
External tools attempting to confirm continued connection with a ping may report Connection timed out - but that is often simply because Sanctuary does not allow pings into Denizen-only Fragments and would be true for a query to an untrapped Denizen still in the Fragment, as well.
An affected Denizen will lose their access to metadata and their net tools, stranding them in the Fragment unless they can make it to a transition zone into another, binding them to the physics of the Fragments, and leaving them with significantly less information at their disposal than they might be used to.
No triggers are known at this time. kraiaKy's Serrata poison may have mutated into a trigger, but this is unconfirmed, and is definitely not the only possible cause.
The condition can be transmitted between Denizens.
The most notable route of transmission is that any attempt to teleport the affected Denizens will afflict the teleporting Denizen with the condition, as well.
Other forms of transmission are currently unknown.