[okiedokieiguess]

lots of time the indicator gets tripped from just moisture not actual water damage. for example if it is in your car and the window is down and it is muggy outside, if you have your phone in the bathroom as you take a shower, if your clothes get wet from rain and then you put your phone in the pocket. Countless things can trip the sensor other than direct water damage.
Have you ever gotten a replacement for your phone before? Or is this the one you bought retail? If it was a replacement the internals are actually refurbished so it is quite possible the internal sensor was tripped before the device was refurbished and just never swapped the sticker. That could be the case if the external one was not yet tripped.
I would recommended bringing it to a different store to see if they can order a replacement, lots of times the store wont even open the phone before ordering a replacement, if that is the case you wont have to worry about the internal sensor and you may get to trick the system.
Best of luck!