New AirTag lost in house, "found" but not "reachable"

To add a bit more ... The "unreachable" message basically means that your iPhone knows the last known location of the AirTag via the Find My network (or possibly your phone’s own Bluetooth if it was nearby recently), but it’s not currently in Bluetooth range — or it is, but it’s unable to establish a stable connection. The reason for the latter could be where it is located is preventing the tag's Bluetooth's signal from going very far ... so your phone would have to be even closer to get an update or for you to hear it.

Regardless, here are some things you can try:

• Use a second device if available — maybe an iPad — to help triangulate the tag's location.
• If your iPhone supports it, go into Precision Finding mode and walk slowly around each room, even if it says "Not Reachable" — sometimes it’ll pop in briefly when you get very close.
• "Kill the lights and go silent" — then hit "Play Sound" and listen carefully. Sometimes it’ll still chirp even with the unreachable status if the connection momentarily succeeds. When trying to hear the tag’s little chirps, especially in a house with ambient noise (HVAC, appliances, etc.), making the room "dark" can actually help your brain focus more on audio cues. It's a sensory prioritization trick: when you remove visual distractions, you’re sometimes better able to pick up on faint sounds. Think of how everything seems louder when you're in bed in a pitch-dark room — same idea.
• If all else fails, use a Bluetooth scanner app like LightBlue (on iOS) to see if it detects the AirTag's BLE signal and gives you an RSSI (signal strength).

To build on Rudegar’s reply:

The location displayed in FindMy was actually the location of the phone which detected it at the time.

The tag itself might have been a considerable distance away from the detecting / reporting phone.

Unfortunately, you’re experiencing first hand why lost pet tracking is an inappropriate application for AirTags.

it was likely the place it was last time it had a connection

it could have gone out of reach since than and the iphone will be non the wiser

the airtags only means of communication is bluetooth so it's only a 30meter range

if no iphone with bluetooth enabled and an active connection to the internet is within range

of the airtag it's dead in the water

from their support pages

• For AirPods and supported Beats headphones, you can locate them when they’re near your device and connected to Bluetooth®. For supported AirPods, you can also see their location via the Find My network for up to 24 hours after they last connected to your device, even if they’re not nearby.

from Locate a device in Find My on iPhone - Apple Support

so wait and see if it's really not where it showed the last location

Do you have an iPhone 11 or newer?

If so, is Precision Finding not working ?

(Click on the “Find” button within FindMy.)

Locate an AirTag or other item in Find My on iPhone - Apple Support