Midrasz do Kapłańska 14:39

2) "in the walls": implying two. And below, (Vayikra 14:39), it is written [(instead of)] "wall," "walls," giving four, whereby we are taught that it becomes tamei only if it has four walls.

6) If in the end we will include a returning plague-spot (even if it does not spread), what is the intent of "And the Cohein shall come and he shall see, and, behold, the plague-spot has spread"? — Leave it (i.e., This is not the place of this verse. It does not refer to post-removal and scraping, but to spreading at the end of the second week, as in 7) below.) Or, once we have learned that if the plague-spot returns and blossoms in the house (even if it does not spread, it is tamei), then, if we see it returning on the same day (of the plastering), it should be declared tamei (and razed immediately)! It is, therefore, written (Vayikra 13:39) "And the Cohein shall return" - (Vayikra 13:43) "and if the plague-spot returns." Just as the "returning" there, is at the end of one week, so, (the returning) here, is at the end of one week.

8) (Vayikra 14:39) ("And the Cohein shall return on the seventh day and he shall see: If the plague-spot is spread on the walls of the house,") "on the seventh": I might think in the daytime or at night; it is, therefore, written "on the day," and not at night.