How do I find a hidden device on my iPhone

What do you mean by a hidden device?

iPhones are devices themselves, they have no hidden devices on or in them in any way.

Why do you think there is even a hidden device anywhere at all?

If you think there's an other device using your T-Mobile service plan, then contact T-Mobile directly and see if there's anything else connected to your service plan.

I have a similar issue where Home says a device has been added to HomeKit but doesn’t show up anywhere.