The location expectations depend on the Find My settings you had on the device: Send Last Location - if the battery is critically low, you’ll see the last location for 24hrs; Find My Network - allows you to see location when it’s offline, in low power mode, and offline, but it relies on other nearby Apple devices. If you’re unsure of which settings you had enabled, we can only make assumptions about the location updates. Things like: SIM card removed (if physical), being out cellular signal range, no nearby Apple devices, phone powered off, etc.

The most likely answer is they removed your SIM and/or powered the phone off.

If Find My shows Offline, No Location Found, etc.: https://support.apple.com/en-us/HT204233