My new phone came with super user but I didn't root it
I'd check with the operator, and see if they root their phones by default (it's a perfectly valid thing to do, it's just that the big operators tend to be control freaks). If they don't know what you are talking about, worry and ask them for help figuring out what's going on.

I'd also go into SuperUser and check it's preferences. Since you don't know how it got there, you don't know what apps might be doing. Turn on it's logging (if not already enabled) and set 'Automatic response' to 'prompt'. That way you'll have a log of what's using it, and you'll be prompted for anything that wants to do things with root access. The idea is that if something asks for root access and you don't know why, just deny it and see what breaks - very few apps should need root access, so it shouldn't come up very often.

If logging is already enabled, then you can review the log to see if any apps have been using root access.