I don't see anywhere where he mentions they are going to send or "return" BTC to you... they're simply explaining about Bitcoin "change" outputs generated during a Bitcoin transaction.
In my opinion, the two things (email from BitPay support and the unknown transaction) are merely coincidental and not related. If you did not initiate the transaction, then someone else has access to your wallet.
This would likely be because someone has access to your 12 word seed mnemonic... Did you ever store your 12 words in a digital format? (screenshot, typed in a text document, sent via instant message or email etc)? Did you ever type it into a website to claim forked coins? Did you ever give those 12 words to another person?
In my opinion, the two things (email from BitPay support and the unknown transaction) are merely coincidental and not related. If you did not initiate the transaction, then someone else has access to your wallet.
This would likely be because someone has access to your 12 word seed mnemonic... Did you ever store your 12 words in a digital format? (screenshot, typed in a text document, sent via instant message or email etc)? Did you ever type it into a website to claim forked coins? Did you ever give those 12 words to another person?
I have give nothing to anyone, when i have imported my wallet it was on Bitpay too.