Thumb sucking

So i used ferbers method to sleep train my baby and it worked really fast. In method they suggested to get rid of pacifier and we did and baby didnt seem to have problem with that either. But last few days i caught him sucking his thumb, i dont know whether this should concern me. I tried just moving his hand and not paying too much attention to it. He is teething these days so maybe thats why. Any moms had experience with this now or before?