This interesting marking that can be seen on various doors around the city is usually put up promptly after the new year and left for the rest of the year. The numbers represent the current year, while the C+M+B are the three wise men Caspar, Melchior, Blathazar.
It's interesting to see this small blessing, because although Bavaria is very Catholic, it seems as though few are actually practicing.
[edit: Reader Lief says C+M+B actually means "Christus mansionem benedicat" which means "may Christ bless this house" - my sources were wrong. Thank you]