When Luke is hiding from Vader under the throne dais, the consoles he’s using to conceal himself don’t even reach to the ground. When he screams “No!” he’s right out in the open like Vader couldn’t see him. there needs to be more of a maze of consoles and equipment underneath the throne.
I think Vader was just toying with him. He was too busy tempting Luke to bother finding him. He doesn’t want to kill him.