When it beeps, note the green and red lights. There's a description on the face of it describing what it's alarming to, power on, CO alarm, propane alarm, low battery, and end of life.
Do either of you use any aerosol product before it starts alarming? Mouth wash that contains alcohol? If it's the propane side alarming, there may be a minute leak that it's detecting, but you may not be able to smell. They can be very sensitive.
We had one that alarmed randomly, in a closed pop up in our driveway. No reason we could find, except for possibly the odors from the wood and canvas inside. We put a new one in, and it still did it.