How do I tell if lost and/or stolen phones were erased and re-activated by someone else prior to locking in Find My iPhone?

My two sons both lost their iPhones a couple months from each other. We waited a solid week or two before putting one in lost mode and potentially longer for the second because we didn't know it was lost. We're not sure if they're actually in the house mis-placed or actually lost/stolen right now. Unfortunately, I was unaware of the newer option to have an iPhone tell you the last known location so I'm unable to see that when I login to 'Find My iPhone'. Both are still in 'pending' status as lost.

Is there anything Apple or my Network provider could do and/or tell me about them by chance? IE: If someone else has activated them for example to know if they were recovered by someone else?

I'm assuming we're out of luck at this point but wanted to exhaust all options before we give up.

Thanks!