Let's talk about Hunts - a detailed rundown of their assumed mechanics

[Neuflune]

i did notice that you have to be in the area for a good 3-5 seconds before kaiser will spawn (it was my last location, so i was chatting to say negative, but it spawned on me when i stopped for a few seconds).

I can confirm this. Perhaps it can be attributed to lag times or flying (as opposed to being on contact with the ground). Both times that I have spawned Kaiser Behemoth, the S rank appeared just as I left the area; unlike Nandi which appears as soon as I step on a spawn point.