Kitten with mouth open

I have adopted a 5 1/2 month old kitten who was neutered last week. He goes around smelling things, furniture, the other cats and puppy with his mouth open. Its almost like a silent hiss but he will walk around like that. Any idea what that is about? He is the big boy in this picture.

My guy would smell and have his mouth open in spots where he sprayed pre-neuter... it was actually helpful for when I couldn't pinpoint where to clean That's the only time he would do that, and he hasn't done it since. *knock on wood he was neutered last week as well*

They do that to smell things more definitely. He's very interested in what he's smelling.

There is actually a name for it!
Flehmening occurs when your cat smells something and then opens its mouth slightly, wrinkles his nose, and curls back his upper lip. The ears also become flattened in some cats. This apparent grimace is also called the 'flehmen reaction' (sometimes also known as 'flehming'). The cat draws in air, capturing the scent, and transferring it to a small specialized sac called the 'vomeronasal organ' or 'Jacobson's organ.' This organ is located high up in the roof of the mouth, and has a large blood supply. It traps the odor molecules and sends signals to the brain regarding the scent.

I've always heard that cats have a 2nd set of scent recievers in the roof of their mouths. When they want to pick up the scent really well they will open their mouths to find it better. I dont know why this would be directly related to post-op kitties, but maybe he's looking for a scent he used to leave and cannot replicate it. Like an old faded mark? Both my guys will sniff with their mouths open occasionally 1 is fixed and 1 is not.