You haven't mentioned negative margins or padding.
Use a div to enclose the iframe and then put a negative margin-left or padding-left in the div. Sometimes this works. It will be a hack as the original cause of the indent would not be found, but it's worth a try. It may move, but may cut off the left side of the iframe if there is some kind of block that Dreamweaver is putting on the left.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.