Found 14th Apr 2017
So my friend was playing on their Xbox One and got a message from Xbox Live. They sent through a code for a £5 voucher. No minimum spend, just £5 credit to spend on whatever you want in the Xbox Store. And they aren't a Gold member and have never purchased anything on their account. I'm a Gold member but didn't get any similar message. Worth checking your accounts to see if you have been sent a code.