Is a caster of Gentle Repose aware that a creature under the effects of Feign Death is an invalid target?

Question

Alice (a wizard) & Bob (class unspecified), are planning an elaborate prank on Carol (a cleric). Alice casts Feign Death on Bob, and then wails about how he has been slain by a horrible monster/curse/whatever. Carol, who is for some reason friends with these jerks, comes running. Carol does not routinely prepare resurrection spells; instead, she casts Gentle Repose on Bob to preserve him for tomorrow, when she will prepare an appropriate spell to bring him back to life.

Is Carol aware that Bob is not a valid target for the Gentle Repose spell (since he isn't actually dead), either before or after attempting to cast the spell?

From the PHB, p140:

Feign Death
...
You touch a willing creature and put it into a cataleptic state that is indistinguishable from death. For the spell's duration... the target appears dead to all outward inspection and to spells used to determine the target's status. ....

From the PHB, p215:

Gentle Repose
...
You touch a corpse or other remains. For the duration, the target is protected from decay and can't become undead. ...

Answer 1

Xanathar's gives a concrete description of the outcome of such a case.

If you cast a spell on someone or something that can’t be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended. If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn’t attempt one (giving no hint that the creature is in fact an invalid target). Otherwise, you perceive that the spell did nothing to the target.

However Xanathar's itself openly expresses that it is optional rules.

The material in this chapter is meant to make your life easier. Ignore anything you find here that doesn’t help you, and don’t hesitate to customize the things that you do use.

So a DM may certainly decide to rule differently. Although it seems the logical outcome.

Answer 2

The description of Gentle Repose does not say it tells you if target is dead or not. There is no mention of "glow" effect or any feeling or anything like that. Therefore, it will not reveal to you that information.

If DM rules that Gentle Repose has a perceivable fluff effect, and the spell is used to determine if target is really dead, this should fail per description of Feign Death. In this case Feign Death should feign the effect.

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It is possible that Carol notices Gentle Repose did not actually work (this is up to the DM), but they don't have a way to know why. If they don't know of Feign Death, then they will be rather flabbergasted probably. Has Bob already been made into an undead? Is the corpse actually a fake/construct/soulless clone/tangible illusion? Is there some monster ability or evil spell which blocks Gentle Repose somehow?

If I were the player of Carol, I would burn the fake corpse and go look for real Bob.