Hi Rich, with thousands of installs, we’ve had probably 10 reports of “empty syringe” but in all cases the chip was in there. As an EMT you know - every body is different, so it may just be a bit too deep to feel properly. Another possibility, however unlikely, is that it has migrated somewhere else… but typical migrations are rotational realignments, not really lateral progression. An x-ray is, of course, the fastest way to find it… hopefully you have a buddy who work in radiology?
What are you trying to scan for it with? Have you considered trying an ACR122U? They aren’t perfectly awesome, but typically way better than a phone at getting a good read… as long as you try different orientations of course.