I am posting to this to share our recent experience, and hopefully help others who might experience what we did.
We were driving on an extended trip on the highway in our 2017 Model X when the "door warning" alarm on the right rear FWD went off. We immediately assumed our daughter had pressed the button, but she adamantly denied it. It happened again a few times while I was seeking a safe place to pull off to the side of the highway. I checked the door, looked for obstructions, wiped off some of the latch area, etc. We took our key fobs and placed them in the center console to avoid any chance we might be inadvertently pressing on them. However, the alarm continued to go off repeatedly during the next 4 hours of driving. With no solution available, we just had to endure it.
We were subsequently stuck in stopped traffic due to the snow, so I started browsing the web to diagnose the issue. I found a similar post describing the symptoms, including one where the own returned to his Model X and found a FWD open even though they knew it was not open when they left the car. Thinking back, we had a similar experience where we were called out in shop to state that "the door on your red Model X is open". I had always assumed this happened by accident pressing of the key fob in my pocket. However, in the thread I was reading the owner received a call back from Tesla Service and was told that the car logs showed the door opening command coming from the key fob.
On a whim, I took one of the key fobs (we had two with us) and removed the battery. The other key fob remained in the car with the battery installed. Prior to this battery removal on the key fob, the car was alarming every few minutes - always from the same door. Over the next two days, it never happened again. After those couple of days, put the battery back into the same key fob and we haven't seen any issues over the week since.
Just wanted to put that out there that if you experience this issue, there's a chance the key fob is the culprit.