Can a hidden GPS be installed on an iPhone?

No! It would require the app on your phone and you would have to give it permission to access your location. If someone has access to your Apple Account with the Password, they can view your location. If you think your account has been compromised, then reset your Apple Account password.

If you think your Apple Account has been compromised - Apple Support

As long as you are using Two Factor Authentication, another device cannot log into your account without the verification code sent to a Trusted Device or Trusted Phone Number that is on your account.

Mikemeg wrote:

Can a gps (app, or something) be put on my iphone with out it being detectected

Carriers have sold location data among other possibilities. Read the fine print of the contract for what they can do nowadays, too; what is (or is not) included there for consent.

If you're an investigative journalist, political dissident, senior on private or government, with access to great wealth, with access to sensitive or classified data, active in defense or military, or have annoyed somebody seriously rich, there are other possibilities.

Another possibility involves a password or passcode comprise, and there are other possibilities.

What issues or concerns or oddities are you encountering, and for how long?