A Ghost (MM, p. 147) can possess a humanoid as per their Possession action:
Possession (Recharge 6). One humanoid that the ghost can see within 5 feet of it must succeed on a DC 13 Charisma saving throw or be possessed by the ghost; the ghost then disappears, and the target is incapacitated and loses control of its body. The ghost now controls the body but doesn't deprive the target of awareness. The ghost can't be targeted by any attack, spell, or other effect, except ones that turn undead, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies. […]
A paladin's Divine Sense (PHB, p. 84) says:
The presence of strong evil registers on your senses like a noxious odor, and powerful good rings like heavenly music in your ears. As an action, you can open your awareness to detect such forces. Until the end of your next turn, you know the location of any celestial, fiend, or undead within 60 feet of you that is not behind total cover. You know the type (celestial, fiend, or undead) of any being whose presence you sense, but not its identity (the vampire Count Strahd von Zarovich, for instance). […]
If a humanoid was possessed by a ghost, then a paladin were to enter the room and use their Divine Sense, would the humanoid register as undead (or the undead nature of the ghost still otherwise show up on the paladin's "radar", such as detecting undead within the humanoid or something like that) or would the possession effectively render the ghost immune to being detected by Divine Sense?
Best Answer
RAW indicates No.
The phrase
in the description of Possession, together with the following listing of untargetability indicated that the ghost for the duration of the effect does not exist as an independent, locatable entity distinguishable from its host. Therefore, because the host is not undead, and the ghost does not have a location (as written, it disappeared), the Divine Sense can't locate the creature.
In a different interpretation, the ghost could be said to be inside the human in question. However, in this case, RAW still would prohibit it, because Divine Sense allows you to
(Emphasis mine) Being inside an opaque object, the human body, certainly would count as total cover, and therefore stop the detection.