2014 GT. In addition to the normal key fob functions, this also has the passive entry/exit with auto-lock (locks when you walk away from the vehicle) and the unlock button on the two doors. Wife was complaining that all this functionality was becoming more and more erratic. We have the two fobs, with one in daily use, and the other on a hook at the house as the backup (for those rare occasions where I need to drive it, for example).
Her initial assumption was that the daily-use fob's battery was dying, so she swapped fobs. But it also was erratic, to the point where it was nearly impossible to get into the car. A few times she had to use the actual key (a key, how quaint ), which of course sets off the alarm until she gets the car started.

I finally got to investigating myself, and a quick battery swap (CR2025) in one fob brought it back to life. In a while I'll go and buy a second for the other fob.

So it appears the "static" battery life in the fobs is almost exactly 3 years, and that when they die, they'll all die at the same time (roughly). I have a vague idea how the passive entry fob technology works, so I know the fobs will always transmit occasionally when within range of the vehicle. Since both usually are, that's the only logical explanation I can come up with for both dying at pretty much the same moment.

Take-away: Get yourself a couple of spare CR2025 batteries (recommend the good lithium ones) and keep them handy, because it appears when one goes, the other isn't far behind, and it's a royal pain in the a**.

Brad.