CoD 4 and MW2 Map files found in Modern Warfare 3?

So Reddit user SMKTZ found five maps from the original Modern Warfare and it's second iteration hidden away in Modern warfare 3's game files.

This has been a historically regular occurence with the previous titles in the series, with file extension-related 'hints' to recreated homages to previous famous multiplayer maps of old.  This time around, we have 4 maps from original and one from the second: Crash, Crossfire, Strike, Overgrown and Highrise.