Alley's sister used to do that. The only time any actual sound would come out of her mouth is if you tried to pick her up or if she got shut out of the bedroom. Otherwise she would open her mouth and make a little squeak noise.
If we were cats, we'd hear them making a noise when they did this. Predators of small rodents tend to be able to hear (and produce) noises of a higher pitch to help them detect their prey, than we can hear as humans.
Baby does that. She is almost 19 and very rarely makes a sound when she meows, now when she howls..... you can hear taht from a mile away! Too funny! guess she knows she needs to speak up when she wants to be heard!