If not the battery, is your fob behind your smartphone? Smartphones can block signals. I sometimes get a "fob battery low" warning, but usually my phone is shielding the fob from the sensor (wherever that is).
Or, most likely, as @BooMan 62 says, it could actually be a low battery. Some dealers replace the battery free as a good-will gesture, others charge a pile of $$$. The battery is nothing special. You can get a new one at Radio Shack, Batteries Plus (they'll probably install it for you), or most any hardware store for a couple bucks.
If this is happening with both your keyfobs then you might have an issue with the radio receiver in the car. Try fresh batteries in both fobs just to get that out of the way, make sure the fob is not behind a phone as others said or in a purse/pocket that has RFID/radio/anti-skim blocking feature. If you are holding the fobs in the open air and they are not detected with fresh batteries probably will need to be checked out at the dealership.
Does your passive entry still work? Lock/unlock buttons?
Also note that when the trunk lid is open it may report that the key fob is not found or has left the vehicle. This is because there is apparently an antenna in the trunk lid that is used in triangulating the location of the fob (inside the car or outside the car).
When the fob is not detected due to low fob battery you can still start the car by pressing the engine start button with the tip of the key fob itself. It has a backup RFID chip that can be read by a sensor in the start button assembly.
Generally speaking if fresh batteries in the fobs don't help and you eliminate simple things like the tailgate being open or phone/purse blocking the fob signal then you will need to bring it to the dealership and let them diagnose it.
I take my fob to batteries + and purchase the lifetime battery replacement ( about $20 ). It pays for itself. My other car (VW GTI) goes through fob batteries about twice a year. I purchased my used trail hawk in January and have already gotten the low battery warning.
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