How to find out who's stealing my Wi-Fi?

So i have comcast internet and we get monthly 250 gb of data.a month my family uses about 110 gb of data. That includes xbox live for about 3 hours a day, 4 iphones for fb and email. 2 laptops for light surfing. The reason why i know someone is using my network because on the 7th of this month my data usage was up to 34 gb already since. I changed Wi-Fi password, my said is hidden and i'm using wpa2.since then i've told my family to tone it down on the internet usage, i have not even turned on my xbox or hardly used my laptop. I checked my data meter today around 6pm and it was at 36gb, check it again just now its 38gb. How is this posible when the most it was used for was my sister using a text free app no way that can use a whole gb of data. So how can i find out who's stealing my Wi-Fi? Does my router use data just by being connected to the modem?

Added (1). Well usually the data meter is spot on same as when i call comcast and ask them for my data usage as of date. My router does have a great range i mean i can walk up past 2 houses and ill still have one bar still.i've noticed one of those houses close to me have no Wi-Fi either the one next to me or the one across the street. Any programs that can help me find out, i've checked my router to the devices attached to me and those are all mine.

I doubt it's someone using your Wi-Fi.

Even if someone managed to hack your password the first time, when someone changes the password, people get discouraged.

Since its home Wi-Fi, it'd have to be a neighbor. And most people either A. Have internet in their homes or B. There's another house within their range with NO security that they'd use instead.

So it's more likely that there's just extra usage you're not accounting for.

Also note that whenever you go to check data usage, it's usually delayed (as in, when you check, that's what data was at a few hours ago)

You will not find out exactly who it is unless they used their name to name their pc. Your router should allow mac filtering, allow all your devices in the mac filtering page, any other device trying to connect will now not be able to but i would have thought changing the password as you have done would stop anyone from connecting. It is possible you are using more data than you might have thought, streaming hd video uses a lot of data, even normal youtube videos can soon mount up the data usage

Change your password once more. Mac filtering is a good idea. Hide your SSID as well. That will make sure only your household would know there's a Wi-Fi in the house. Nobody else can connect if they can't detect the SSID.

Also, I don' believe there's any unauthorized user of your Wi-Fi as you have said you recognize all devices when you look at the router's list of connected devices.