Marc17
Explorer
I'd say the call for initiative comes with the declaration of attack. From there it just gets into how the DM wants to narrate it. Perhaps somebody saw it after it lept. Perhaps was getting prepared to leap but wasn't hidden enough. Perhaps it developed the death lust with the intent to attack and another felt that (popular in Lone Wolf and Cub manga).This can get alittle muddy, though, from the perspective of who can see what when their initiative comes up. If the leopard has not leapt out until its turn, it is still hidden when the PCs go. What can they reasonably do, without using player knowledge, then?
Still, if the DM doesn't like it or players protest enough, just change it. My ongoing opinion is that the rules are purposely written sloppily to force the DM to take action on them. Why else would we have Hiding make you Invisible, which doesn't actually hinder detection just the targeting of spells on "Invisible" people.

