What's the exact meaning of "flat eyes"?

In the American novels, there are a lot of expression of this 'flat eyes'.

Seemingly not very favorable expression, but what's this phrase insinuating exactly?

I am not a native speaker of English.

Flat usually refers to dull and emotionless.

lacking interest or emotion; dull and lifeless; lacking vitality or animation.

It is often misused, I had to look for a while to find a good example, and this was the best I could do (it's not even a good sentence!)

Maybe because his flat eyes had no expression and he was squinting at me like a mob character.

Flat can similarly refer to affect, scenario, food or joke.