This cable or accessory is not certified and may not work properly with this iPhone, when nothing is plugged in

Hello,

For the last several months I have had a nagging issue on my iPhone 6 Plus. Randomly, throughout the day, I will receive an alert from my iPhone telling me:

"This cable or accessory is not certified and may not work properly on this iPhone" (might not be word for word)

My only option is to dismiss the alert, which is all well and good, but my problem is that nothing is plugged into my iPhone when I received this alert. It happens completely sporadically, some days it won't happen at all, other times it will alert me 5 times in a matter of 2 minutes.

I have tried it with bluetooth off, wi-fi off, in airplane mode. I have tried rebooting, I have tried holding the home and power button down until it resets, I have even restored it to factory settings. I have never had this issue with any other iPhone model, and even my wife's iPhone 6 Plus has come across the issue, but only a handful of times (same circumstances, i.e. nothing plugged in).

The phone works well for the most part, but I am especially annoyed when watching a video, and the alert pops up, which pauses my video until i dismiss the alert.

If anyone could offer me any help, that would be great. I am out of warranty, and its not a big enough issue to buy AppleCare to get it replaced.

Thanks in advance for any thoughts on the subject!