For many reasons:

- Antibody non-specificity - The Flag antibody may be binding non-specifically to other proteins in your cell lysate. Try using a lower concentration of antibody or trying a different Flag antibody.

- Leftover Flag-tagged protein - If you used the same equipment (e.g. same tubes, same blotting apparatus) for your Flag-tagged samples and control samples, there could be leftover Flag-tagged protein contaminating your controls. Be very careful to thoroughly wash all equipment in between samples.

- Vector leakiness - Even without induction, your vector may be "leaky" and expressing low levels of Flag-tagged protein in the uninduced cells. Check your uninduced cells for any expression of your protein of interest.

- Cell line cross-contamination - Make sure your control cells have not become contaminated with your Flag-tagged cell line. Check your cells under the microscope to confirm their identity.

- Non-cleared lysate - If you are re-using lysates, make sure to centrifuged and transfer fresh supernatant to clear any debris/cells that could be carrying over Flag-tagged protein.

- Insufficient blocking - Make sure you are blocking the membrane sufficiently before adding antibody. Increase block time or concentration.

So, the key is to carefully control and validate each reagent and step. Try simplifying by using lysate from untransfected cells only, with fresh buffer, tubes, etc. If the band remains, the antibody is likely the issue.

its non-specific band probably. You can just compare your FLAG tag-protein size with control non-transfected or transfected with control plasmid.

Cross contamination of the cell line maybe.