stress signs in shy cats

I've been working with a few foster cats lately who are really on the shy side, and I'm finding it hard to distinguish between normal introversion and actual stress. Since I'm a vet tech, I know the textbook signs like flattened ears and dilated pupils, but these cats are so quiet they barely react to anything. I'm wondering if we're missing something subtle. Does anyone have tips on recognizing stress in cats that don't show the obvious aggression or hiding?

In my experience with older cats, the subtle signs are often in the routine. My Simba is deaf, so he relies heavily on visual cues, but even he will stop grooming himself if he feels threatened. If you see a cat that usually grooms a lot suddenly stopping, or if their eating habits change, that is a big red flag. I would suggest looking at Appetite Loss vs. Picky Eating for more details on how to tell if they are just being picky or actually stressed.

My cat Jasper is hyperactive, so I'm the opposite of shy, but I've seen the quiet ones. They usually just stare at nothing or become overly fixated on a spot. It's annoying when they don't listen to you, but if they are doing it constantly, it's a sign of anxiety. I think people overthink the body language sometimes.

Keep it simple. If the cat is not using the litter box or is eating less, something is wrong. I have an older cat with glaucoma, and his stress showed up as irritability around his eyes. For shy cats, watch for excessive licking of paws or fur. That is a cheap way to check their mood without buying anything.

@kims73 that makes sense. I was reading Is Hiding Always Fear? earlier and it argued that hiding can be a comfort behavior, not just a fear response. I guess I need to look at the context more. @jgreen, how do you know if staring is just boredom vs. anxiety?