A pen cap for a medication delivery pen includes a piston-style detector mechanism. The piston-style detector mechanism includes at least an inner shell having first open end through which the medication delivery pen can be inserted, a second end opposite the first end, a sidewall defined by an outer surface and an opposing inner surface, and a passageway extending from the outer surface to the inner surface. The sidewall extends between the first end and the second end thereby defining a pen-receiving cavity there between. The piston-style detector mechanism further includes at least one switch and a translatable shaft at least partially disposed in the passage. The translatable shaft includes a body that extends at least from a pen-interfacing portion in the pen-receiving cavity to a switch-interfacing portion thereof. The translatable shaft is oriented to travel from a first location to at least a second location during capping of a medical delivery pen into the inner shell to toggle the at least one switch.