My concern is: for what reason? People don't go around implanting stuff into people for no reason, and I wonder why'd they put it in such an inconvenient spot. At some point, this implant must communicate with the outside world at some point if it is to be at all useful. I suggest some tests:
If it's an RFID/NFC device, would an NFC-enabled phone (or anything capable of reading NFC tags) be able to detect its presence?
If this were Bluetooth, can you not use one of many exploits to just guess at its MAC address and eventually hit a so-called "hidden" device? If not you, get a tech-head to do it for you. Hell, it might not be smart enough to be hidden, and you can detect a Bluetooth device while in a 2.4GHz Faraday cage with just a phone (or other Bluetooth-enabled device) and yourself in it. Make sure you have no other Bluetooth devices on you so that you can narrow it down.
If this were WiFi, does there not exist software that can just scan the area for any and all WiFi networks, including the "hidden" ones?
What technology does this implant use? I'm sure with a nice, very-high-end oscilloscope, you can connect a directional antenna to the 'scope, then point it towards the implant, and use the FFT function to detect peaks in the signal corresponding to RF emissions by the implant. What device is meant to interface with it, if any?
These are all very important questions that need to be asked. If you cannot detect it at all by any means, then it falls into "The Dragon in My Garage" territory, and it can no longer be considered a verifiable belief. When you go up to the courthouse to plead your case, you must have facts to go with your arguments. You (your lawyer actually, but they're following what you say) need to show clear evidence of the implant's existence (by way of detection with other tools), else your case falls apart.
But if Rolan's suspicions are correct, it wouldn't be worth it to argue. It's often impossible to differentiate fact from fiction under these conditions.