That's not a tab, it's a left indent marker, so you need to reset the left indent on the style.
Open the style defintion dialog for the applied paragraph style and go to Indents and Spacing. You'll need to first reset the First Line Indent fro the negative value to 0, then set the left indent value to 0 (you have to do it in that order).
If this is being applied as a local override (the style definition already shows 0 for left indent), you can use Find/Change to find the paragraph style and replace withthe same style, which will remove ALL overrides for the style, or you can look for the indent values and change those (there's an indents and spacing section in find format and change format dialogs).
You should also verify that no indents are being applied when there is no active cursor insertion point, selected text or selected text frame. You can do that in the Paragraph panel or the Control panel if the text tool is selected.