As both an engineer, and a Ham radio operator, perhaps I might add something to the discussion.
While some posts have noted that the body is water, and therefore a reasonably good conductor, it doesn’t explain why the effect vanishes away from your mouth as it does. Indeed, putting the remote about waist high should be the most effective position if that were true.
There is another more viable explanation from antenna theory.
By putting the remote in front of your mouth, you are creating a Yagi type antenna. Briefly, a Yagi antenna will focus radio energy along its axis, thereby increasing the signal in that direction. Starting from back to front, a Yagi antenna consists of a “reflector”, “driver”, and usually several “director” elements. They are made of metal, but insulated from each other. The refector and director elements are passive. Only the driver element generates (or receives) radio energy.
When you put the remote near your mouth, your metal dental work acts as the reflector, the remote as the driver, and the keys as the director elements, creating an ad hoc Yagi antenna. If you don’t have metal in your mouth, eye glasses, earrings, neck jewelry, or even your bare head will work, but probably not as well.
Even a very poor Yagi will double or triple the signal intensity along its direction, thereby explaining the increase in range.
When you open your mouth (assuming you have metal in both jaws), you enhance the “reflector” performance for the wavelengths typically used for car remotes, so opening your mouth could also make a difference.
Sorry Ray, I tried to find some support for the “canyon” effect, but couldn’t find a shred of evidence for it.
Peter Higgins, PE, N6PH (Extra Class Amateur Radio Licensee)