"might have been" vs "might be"

I have a question about the present perfect and present simple. Here's a sample dialogue.

A: "I can't find my umbrella. Have you seen it?"
B: "It might be in the car."

Why don't we use the "might have been" instead of "might be"?

And here is another dialogue.

A: "The man you spoke to - are you sure he was American?"
B: "No, I'm not sure. He might not have been American."

Why don't we use the "might not be" instead of "might not have been"?

A: "I can't find my umbrella. Have you seen it?"

This is definitely idiomatic, but it does include two time-frames, present (I can't find) and past (though recent, of course) (Have you seen).

B: "It might be in the car."

addresses the present situation, and is the standard answer.

B: "It might have been in the car."

is not incorrect, and might be paraphrased as "You know, I think I remember seeing it in the car." It refers back to a situation in the recent past when B was in or near the car.

.............

A: "The man you spoke to – are you sure he was/is American?"

are both idiomatic. Assuming the man still to be alive, they are interchangeable, one focussing on the past context, and one on the continuing state (if he was American, he still is American).

The answer will typically follow the form of the question as regards tense.