In spite of the owner's manual saying we'd get a message indicating weak fob batteries, it does not happen. I intentionally went through the following to see if a message would appear when the fob battery gets weak.
It started with not unlocking the door using the outside handle button with fob in pocket. Then it progressed to not recognizing the fob in the car requiring putting it under the steering column. Finally, it would not unlock the doors using fob button. I was watching to see if/when the message would appear. Result: no message ever.
At that point I replaced the battery. I kept a spare battery with me in case I was out and about when the battery was too weak for anything.