'Lost mode' says pending. What does this mean?

Hi Pia-Philo

welcome to the Apple (user-to-user) Community.

'Lost mode' says pending. What does this mean?

Sorry your watch seems to have been stolen. Activating Lost mode was a great thing to try!

When a device is marked as lost, one of these status notes appears below the Mark As Lost section:

Pending: If the device isn’t connected to a Wi-Fi or cellular network when you mark it as lost, Pending appears until the device goes online again.

Activated: Once the device goes online, the device is marked as lost.

More info: Mark a device as lost in Find My on Mac - Apple Suppor

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If you haven't already, you may want to check through Apple support's online article (link below:

If your device is lost or stolen - Apple Support

Eg: says:

"If you didn't turn on Find My before your Apple Watch was lost or stolen and your watch isn't connected to Wi-Fi, cellular, or your paired Phone, you can't use it to find your device".

and

"Report your lost or stolen device to local law enforcement. Law enforcement might request the serial number of your device. Find your device serial number. "

"Find My is the only way that you can track or locate a lost or missing device.

There is no other Apple service that can find, track, or otherwise flag your device for you".

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Re: "I checked iCloud and I still see my watch under my devices but it’s offline,

I think because it’s not connected to wifi."

Yes. see: If Find My is offline or not working - Apple Support

Eg says:

Why is it offline?

• Your device is powered off or its battery has run out.
• It's not connected to a known Wi-Fi or data network. 
• If you turned on Send Last Location, more than 24 hours has passed since it connected to a Wi-Fi or data network. 
• You might be in a region where we don't offer this feature due to technical limitations or local law. 

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Re: "Also I did have a passcode to the watch. Can the person reset it by any chance?"

Seems the person would have to know or access your Apple ID and password to reset the watch:

You may want to : Change your Apple ID password.

When you change your Apple ID password, you can prevent anyone from accessing your iCloud information or from using other services from your missing device.

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Find My Support Website:

Find My - Official Apple Support

Apple Watch support website: (info / support / contact options etc.

Apple Watch - Official Apple Support

All the best :-)