This is a release of STATIC or a natural build up of electrons (negative charge) from a person in a moving vehicle (friction). After being in motion or causing friction, touching an object, like the key pad, releases the static build up from a person to the object they touch. In the summer, the air is more humid, and static electricity does not build up as much as during the winter, when the air is very dry.
To avoid a shock, use a pencil to enter the numbers in the key pad or purchase a remote control. Please warn your guest so they will not be caught off guard.