Adam Clarke on Luke 11:44

Ye are as graves which appear not - In Mat 23:27, our Lord tells them that they exactly resembled white-washed tombs: they had no fairness but on the outside: (see the note there) but here he says they are like hidden tombs, graves which were not distinguished by any outward decorations, and were not elevated above the ground, so that those who walked over them did not consider what corruption was within; so they, under the veil of hypocrisy, covered their iniquities, so that those who had any intercourse or connection with them did not perceive what accomplished knaves they had to do with.