Gone down below neck. You mean you're able to see the radiator core (the fin tube openings)?
You're checking this on a stone cold engine right? I believe so.
I'm honestly out of ideas. You've checked all the hoses and cooling system components and no leak was found.
Sometimes the two hoses going to the throttle body start leaking due to becoming brittle with heat. But you would've noticed water in the area. Benefit of using coolant is green stuff is super easy to spot when it leaks.
Not sure about the vaccum but even when my throttle body hoses of baleno were leaking, my radiator would still suck up coolant from reservoir. And it also did suck coolant from reservoir when my water body was on it's way out and was leaking from the seepage hole.
I'm no expert but I'd focus on the reservoir area itself. Even with leaks it should still suck some coolant from reservoir.
What type is the reservoir cap? Does it have two pipes? Mehran style ones just have a small hole in the cap itself. Baleno has two pipes and I think cultus should have something similar.
Make sure the hose from radiator is going to the correct pipe. I think it's impossible to put it wrong as their diameter is different.
I'd check that the other pipe on reservoir cap is not blocked. As coolant contracts, the reservoir cap needs to allow air to go into the bottle so it can fill up the space when coolant is sucked to radiator.
