Q: my iPhone was stolen. Will the relevant Apple authorities use my serial number to find my iPhone if/when a new iCloud account that isn't mine has logged in on it?

My iPhone was stolen on 13th April. I reported it to the police but nothing has happened as yet. I changed  my ID password and my phone had a passcode. Since that night, I activated FMI yet had been offline. I was keeping track on it to see if it would show up eventually but nothing more occurred. last night, as per usual, I  checked my iCloud to see the devices that were on the account because that was one way I knew whether or not someone interferred with it, only to see my iCloud's been removed. it confused me as to why my phone was still logged in on FMI iPhone though. If I properly inform Apple serial number and former corresponding Apple ID (which was mine) for this device, will they disclose if/when a new ID logs onto the device. I feel like at this point, I can no longer efficiently track my phone, it'll be offline since my account was removed from the phone but I'd like to believe that there is something that Apple can do for me. My service provider has not been helpful in this situation and that's why I resorted to posting this.