So it's the lead-up to what might be World War III and the enemy is kidnapping and implanting mind-control chips into our people, one by one, to turn them into sleeper agents, to all attack en masse when the signal is given. The victims who have been chipped remember the whole ordeal and try and reach out for help, but nobody will believe their crazy-sounding conspiracy babbling. After a while though, when enough people start turning up with the same plight, their claims start to be taken seriously and an investigation begins. The problem is, they can't quite find the chip in anyone!
The obvious/dumb solution, a metal detector, wouldn't work because, this setting being based on the real-world, there's no iron to be detected in the silicon chip wafers or from the precious metal traces on a PCB. And plus, transistors these days are TIIINY! The chip itself may be so small that it would be difficult, if not impossible, for any external scan to detect it, simply due to lack of resolution. The chip can receive commands though, therefore it can be assumed that it it can also reply too, broadcasting detectable (though potentially very weak, possibly intermittent, and on an unknown frequency) electromagnetic waves.
The victims make themselves available to undergo any kind of non-invasive test or process to help locate the chip in their bodies (so no exploratory brain surgery to find it). What kind of scans or other kind of test could have a chance at locating this within one's body, given the size, material composition, and other real-world constraints? CAT scan? MRI? X-ray? Something outside the box? And for the scans that wouldn't work (like the metal detector), bonus points for explaining why.