[ubuntu] Data remains on HDD after being securely erased

Hi all. I have asked this question on several other places, askubuntu and various linux forums, however I haven't received any meaningful answer to this. If this is inappropriate forum for my question, please point me where I could ask it. So, one day I've decided to verify if a securely erased file is really erased completely from the disk. To my surprise, I was wrong assuming that the file is really wiped and no traces left of it at all, at least in my case it isn't so. I have tried wipe, shred, srm, dd, etc. with various options, all to no avail. I'm on Ubuntu 15.10 x64, the filesystem is ext4 and all tests are done on root partition. The disk is HDD. Here is what I do exactly: 1) Create a text file with a unique text like "00b00nt00xxx123" 2) Erase it with one of the above mentioned tools 3) Open up a hex editor and search for a string "00b00nt00xxx123" throughout the whole disk 4) Multiple instances of "00b00nt00xxx123" is found somewhere at the starting sectors, at one place it is just plain data, at another it is a MIME header + data, at another it is data mixed among garbage. Note that I create file with unique data each time and I do not edit file afterwards. Also, there is no copies of the file anywhere on the disk. [br]Sometimes the file really gets erased, i.e. no traces left of it, but in majority of cases it does not. I'm puzzled. Is this normal behavior of ext4 fs? I'm aware of joualing thing, however I don't know how it really works. It seems like before the file is erased it gets relocated by the fs. Perhaps somebody does know what is really going on? PS. Yes, I know I can encrypt whole drive or create encrypted partition/storage, but that is out of the scope of my question.