From Use the Find My app to locate a missing device or item - Apple Support
- "Don't see your device? You might not have Find My [device] turned on. 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 also can't see your device's location."
If the memo was deleted while the device was logged into your account, see Make a recording in Voice Memos on iPhone - Apple Support
You can recover a voice memo if it's still in the Recently Deleted folder
If you are logged into your account on another device then missing items hsould reappear if you sign into iCloud and activate its features.
Note that if somebody was able to delete items from your account then you have a compromised account and you need to pay attention to that immediately.
Read this document if you think your Apple ID has been compromised. --> https://support.apple.com/HT204145
For iOS 15 or earlier, if you want to see if anyone else has access to your device or accounts, click here --> https://support.apple.com/HT212021
For iOS 16 or later see how Safety Check on iPhone works to keep you safe. https://support.apple.com/guide/personal-safety/how-safety-check-works-ips2aad835e1/1.0/web/1.0
Use the information in this document to check your Apple ID device list to find where you're signed in --> https://support.apple.com/HT205064
Related materials:
Personal Safety User Guide
https://support.apple.com/guide/personal-safety/welcome/web
> open the Table of Contents and review the articles
A document with general information about security and your Apple ID --> https://support.apple.com/HT201303
Contact Apple for help with Apple ID account security by clicking on this link --> https://support.apple.com/HT204169 "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues."