Stolen iPhone and can't see on find my iPhone
If the phone disappeared from Find my iPhone on iCloud (which is different from being offline), someone must have removed it.
Even if the thief turned it off and reset it, it would remain on iCloud as an offline device until the account owner removes it.

On a side note, if the IMEI lock disables data on the SIM, it makes it difficult if not impossible to find the phone unless the thief goes to a place where the phone has connected to a wifi network before.