You have brought up a real concern. This just recently happened to me. I normally keep the fob in my pocket with my other keys. It seems that somehow, probably when sitting, a key must have been in position that as I moved it pressed the remote release on the fob twice and the tailgate opened. Not sure if it happened while driving or when I was stopped at a traffic light. But it did happen while in transit. Fortunately all I had between the bed divider and the tailgate was my hitch head and reducer, both of which stayed in the truck. This could have been a disaster as the head unit is very heavy and if it had bounced out and hit a following vehicle could have caused an accident. I was particularly surprised to see that there is not a dash light to warn when the tailgate is open, similar to the door ajar light. I have since transferred the fob to a pocket on my coat away from everything else. This sucks as during the summer the only pockets I normally have are in my pants. So I am trying to see if there is a sleeve I can keep the fob in so as to prevent it opening the tailgate. So yes, your concern is a valid one!
You should look into this further with some testing and also check how well the tailgate is closing. I know the overhead button will not work in drive; had to shift to park to let the GF put something in the back real quick. Not sure on the keyfob though.
I do set off the panic semi-regularly if the keys (several rings on a carabiner) are in my pocket and I squat down or things like that. Normal sitting hasn't done it often.