Hi there emonson,
Thanks for connecting with us in Apple Support Communities. We understand you're asking how to find an iPhone that's powered off and no longer showing a location in Find My. We're glad you're here, and we can provide some guidance.
"If your device is powered off, the battery has run out, or more than 24 hours has passed since its last location was sent to Apple, you can't see your device's location." Keeping that in mind, it sounds like you may be seeing the last location and the device may be powered off. Have a look at this support article to learn more:
Use the Find My app to locate a missing device or item
"If the device can’t be located: You see Offline under the device’s name. The last known location is displayed for up to 24 hours. Select “Notify me when found” to get an email when it’s located again." Learn more here:
Locate a device in Find My iPhone on iCloud.com
Also, you can follow the additional steps in this link to help protect your data:
If your iPhone, iPad, or iPod touch is lost or stolen
We hope this information is helpful. Don’t hesitate to let us know if you have any more questions.
Take care.