When a warlock dies, if they have a Pact Boon item from their Pact Boon class feature (e.g. a Book of Shadows from Pact of the Tome; or a talisman from Pact of the Talisman), it crumbles to ash. Similarly, if they have a pact weapon (from the Pact of the Blade), it disappears when they die. I presume that, narratively, that's the patron's doing.
Interestingly, a familiar (from Pact of the Chain) does not vanish upon the warlock's death.
The description of the 3rd-level spell Feign Death says (PHB, p. 240):
For the spell’s duration, or until you use an action to touch the target and dismiss the spell, the target appears dead to all outward inspection and to spells used to determine the target’s status.
Mechanically, when Feign Death is cast on the warlock, is the fate of these Pact Boon items the same as if the warlock had died? Or does the spell make it look like that Pact Boon crumbles/vanishes?
Importantly, do the mechanics of the spell change depending on whether the patron is fooled by the spell as well?
On the one side, it's going to be a pain to spend an hour re-summoning the book/talisman etc. just because Feign Death is cast on your warlock. On the other, it's going to make it quite obvious that the death has been faked if it doesn't cause those items to disappear or turn to ash.
