I have had this on two occasions where the alarm keeps going while I am driving (2013 s coupe). I have the push start ignition without needing to engage the key in it, and usually unlock the car by relying on the key fob in my pocket. While this is happening, I have turned the ignition on and off but it didn't do anything. I assume the car thinks it is stolen and is displaying the alarm for that. It eventually goes away - but I am not sure what causes it or solves it, it may be locking the car from a distance with the fob that solves it. The last time it happened while I was driving the alarm would go off repeatedly with short interval rests and during this time the turn signals would not work. I was delivering a late pizza, so that is why I continue driving.

 

I've had the car go off 3 times since Aug 2013 when we got it. Twice was my doing (reaching over and grabbing something on the seat while the window was open and the car was locked). But one time it went off while the key was in my pocket.
The key in your pocket may have triggered the alarm. I found out it can only be stopped by placing the key in the ignition. There is no alarm button on the key fob but I wonder if there is a alarm sequence?

edit: I found the answer in the manual. If the boot button is held pushed for more than 2s it will trigger the alarm. It's called "panic mode".

 

Thanks for the tip. This has happened to me twice; most recently just a week ago.

 

+1 "If the boot button is held pushed for more than 2s it will trigger the alarm. It's called "panic mode"."

 

I never knew this! Explains why I set the alarm off on my Roadster now and again while putting stuff in the boot.

 

You are welcome.