When a suture knot is visible below the skin several months after abdominal surgery are there any non surgical methods to make it less visible?

ASK or SEE Surgeon. Suture material can be absorbable or not absorbable. Absorbable suture is usually 'taken up by the body' in the matter of weeks to months. You may just have to wait a little longer. Of course non -absorbable suture will always be there (holding things together) , sorry.
Stitches left on. With sutures it is better to live them for long periods instead of removing them too early. If there is a persistent knot from a stitch it probably won't cause any problem except a granuloma engulfing the material causing like a callous. If it is superficial a dr has to see it to make sure he/she can remove it.
Minor Surgery. Chances are likely that the knot you are feeling consists of absorbable material and will be gone within the first year. If it was permanent suture material, the knot can be removed after one year, under local anesthesia in your surgeon's office.