I'm sorry to hear that you've lost your device. If you have enabled Find My Device on your Xiaomi 15, you may be able to track its location through the Mi Cloud service. Here's what you can do:
1. Go to the Mi Cloud website (i.mi.com) on a computer or another device.
2. Log in with the same Mi account that is linked to your lost Xiaomi 15.
3. Look for the "Find Device" option and see if it can locate your device on a map.
If you haven't enabled Find My Device or are unable to track it through Mi Cloud, you may want to consider contacting Xiaomi customer support for further assistance. They may be able to help you with locating your device or providing guidance on what steps to take next.
I hope you are able to find your device soon. Good luck!