Things to look for to check if your iPhone has been hacked and follow-up needed to secure device with an understanding that ones iPhone activities and location can be remotely viewed and accessed by a potential hacker.

Your iPhone cannot be hacked. So the rest of your post is meaningless.

There is a recently reported vulnerability when using public Wi-Fi that can be fixed by updating to iOS 14.4. Beyond that there is no further action you need to take.

Having direct access to your device and knowing both your Apple ID and password is not in any way shape or form, "hacking". It's no different than if you were using the phone.

From there, yes, the person who had your phone can do anything to it they want to. Just as you could. There are multiple tracking apps on the App Store. For all you know, they could have jailbroken the phone and put software on it that hides its icon from the desktop.

You really have no choice but to entirely erase the phone back to factory condition and set it up as new. Login to your Apple account so you can retrieve your purchased apps and/or music.

You also say this has happened before. I don't think it should be necessary to say you need to keep better track of where your phone is. Much more importantly, change the password of your Apple account and the 6 digit passcode to keep whoever is getting hold of your phone out of it and your account. Do not tell anyone else what either of these new passwords are.

SonnyBahia wrote:

What I mean by hack is that someone has obtained this information because of ones carelessness. They had physical access to the device, knew the Apple ID & password, and successfully downloaded whatever spyware or tracking application. This is very much a possibility and has probably happened before, appreciate your kind thoughts.

That's not "hacking" your phone.

That's like saying you gave someone the keys to your house and permission to enter and when they used it, they "broke in". They didn't. You let them in willingly.

If what you describe had happened restore your phone using DFU mode→How To Put An iPhone In DFU Mode, The Apple Way and to not restore your backup. This will set up the phone as new.