Learn about SppExtComObjHook.dll
As for innocent users, you have no chance to see SppExtComObjHook.dll but it may have run itself on your pc for months. SppExtComObjHook.dll is a typical trojan which is good hider. Sometimes it fakes as a txt, and the other time it is a html, zip, and so on. When you detect this files, you will not plan to delete it and you will think they may be important. So SppExtComObjHook.dll survives.
Also, SppExtComObjHook.dll will run itself with some .exe files or .com files. And it will be packed with them, so when users get them run, SppExtComObjHook.dll sneaks into the system. Smart as it is, it will also do some prevention when users opens the files and detect some things. And it will pop up a error notice to alert you that this documents is broken and cannot be open. Things like this will make it survive for a long time on your pc.