[Jen Lavere]

I need some help at tracking medical devices lost or stolen without a tracking device on it already

[Rukbat]

If it's an HTC cell phone, it has GPS, so you can install any of several tracking apps that make use of the GPS data. (Make sure the legal owner of the device has signed off - literally - signed a piece of paper listing the devices by IMEI and giving you the right to track them - before doing this. Tracking devices owned by people who didn't give written permission is illegal.)

If it's not a cell phone, it depends on the device. (Normally any modification to a medical device voids its certification, so you can't just add a GPS unit to it.)