A longstanding riddle for fans was introduced in the second season episode "Invasion." At the end of that episode, it was revealed that one of our Sliders had been implanted with a tracking device so that the Kromaggs would be able to follow them. This riddle was thought to be solved near the beginning of season three when Arturo suddenly developed a mysterious tumor. Since Kromaggs used biological technology (as observed by Arturo and Quinn in "Invasion"), it was believed this "tumor" was the tracking device. That very well may have been the original plan (since Tormé thought up both the tumor and the tracking device as storylines), but with John Rhys-Davies' departure, any possible plans in that direction would have been scrapped.
So, this still left the question...who has the tracking device? In season four, we got a definite answer. In "Common Ground," a secret Kromagg transmission revealed to the audience that Quinn had been implanted with the tracking device on Earth 113 so that they could track him back home...not to Earth Prime, but to his real home...the Kromagg homeworld. Of course, this may have revealed something more subtle in the process...
Have you ever asked yourself where the Kromaggs put the tracking device in Quinn? We know it was implanted (which means it was surgically placed in some point of his body), but where? Let's think like a Kromagg for a moment, shall we? (Repress all thoughts about eyeballs please...yuck.)
The Kromaggs obviously felt it was of greatest priority to track Quinn to his destination (after all, they want to find their homeworld again above all else), and this would affect where they would place the device. You see, the Kromaggs would not want to risk the device accidentally being separated from Quinn. Therefore, they would reasonably not place the device in his appendages (arms, legs, hands and feet) since those can easily be lost without causing death. That only leaves two areas...the torso and the head. For either of these parts to be lost, Quinn would be dead and couldn't do them any good anyway.
Now, either of those places left are reasonable...but let's look at something seemingly unrelated for a moment. Many people noticed a subtle but obvious change in the way Quinn acted when the third season came around...and that difference only became worse as time went on. Quinn became more of a swashbuckling hero, more quick to fight, largely less sensitive and perhaps even a little less intelligent. That tradition carried on into season four, where Quinn at times became quite sarcastic and condescending (eg. Slidecage)... What could have caused those drastic changes in personality and character?
Well, I believe the tracking device offers us an answer. The episode "Invasion" occurred at the end of season two and was meant to be the cliffhanger for the season...immediately after that, we had the change in Quinn which grew worse over time. Now, of the reasonable locations for the Kromaggs to place the device...one was the head...or more specifically, the brain. As we find out, the Kromaggs did not care about keeping Quinn's intellect or emotions intact...they only wanted him to keep the parts of his memory that could lead them home and his ability to move...the rest was superfluous. So with the advanced science in the Kromaggs' possession, what if they placed the tracking device in the one place where they knew it would never be separated from Quinn and would be unlikely to be discovered...Quinn's brain? They could have pinpointed it to be placed in an area that they deemed to be unimportant to them (his emotional and analytical centers) just in case something was inadvertently altered during the operation or the functioning of the tracking device...and apparently those contingencies occurred, judging by Quinn's actions. Once again...something to think about...