Factory Reset, Google Photo's recovered old photos Recently purchased a Oneplus One second hand from eBay.
It was factory reset by previous owner, and it has since been factory reset by me a couple of times.

I have setup Google Photo's today and it's pulled in all my photo's like it should. However, it has also resurfaced all the previous owners photos and is currently backing them up to my Google Photos account!

When i look at the info on the images, it seems it's dragged them in from 'storage/emulated/0' 

I wouldn't normally be bothered by something like this but these photo's contain enough information for some serious fraud/identity theft! 
I have pictures of addresses, passports, credit cards, tax bills .... 

How is this possible? Surely a factory reset should wipe all data? 

For clarity, the phone is running stock Android 5, but is rooted (supersu) and running TWRP (both installed by previous owner).
A factory reset deletes the files on your internal memory, but does not secure delete them - or delete anything on your internal SD card.

When you press "Factory reset", it quick formats /data/ and /cache/. If you select "Also erase the SD card", then the internal SD card will be erased too.

If they can still be found with Google Photos after selecting the option to erase the SD card during a factory reset, try going to Settings > Storage and then select "Erase SD card".

If you really want to fully erase everything, use a secure delete program (it will fill the internal memory with a file filled with 0's and then delete the file), so you can't recover deleted files.