Pop-up Encounters

Some games have these set at specific locations, others have them appear anywhere (overlapping with random encounters).

Although this could include enemies that are concealed "visibly" (you can learn to identify some visible marker that foretells of their appearance), this however is not for such cases.

This does include cases of enemies that pop-up after a sort of a warning sign, such as rustling in leaves or such, but not if the enemy can be spotted long before you're in the area.

Unlike cloaked NPCs, these can't be defeated before they "pop-up".

Unlike unseen encounters, player may still be able to maneuver out of these depending on the game and circumstances.

This is a variant of Seen Encounters. It is strongly implied that the player has a moment to prepare for and perhaps even avoid the encounter when seen.

Related:
* NPC teleporting - may explain it, but they still pop-up without warning.
* NPC (re-)spawning - may occasionally give an illusion of pop-up monsters.

Similar:
* Cloaked NPCs
* Burrowers