i consider any thread that shadows through the fabric of a quilt a varicose thread. 9 out of 10 times, i notice it after i get right up on it with my machine.
(p.s. - judges HATE varicose threads and seam shadows!)
it might take a couple times to coax that thread out, but the end results are worth it.
i've used this method to bury short thread tails from knots, to hide the tails of embroidery/applique threads, and to flip twisted seam allowances.
now, if it were only that easy to get rid of varicose veins....