Android data usage shows significant usage even without SIM card
The Data Usage screen can show both Mobile Data and Wi-Fi usage (but only shows one by default).



You're almost certainly seeing your Wi-fi usage, 750MB doesn't sound unlikely for a new device that will have downloaded settings, apps, contacts, and probably OS updates.

If you have a SIM card in your phone then the Data Usage app defaults to only showing Mobile Data usage, but there is a menu option to allow you to set it to show Wi-Fi data usage too.



I assume that if the device has never had a SIM card inserted, then it will default to showing Wi-Fi usage, as that's the only data usage it's used.

(screenshots from a Galaxy Nexus, so it should work the same way)