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.
These probably form part of Infinity Ward and Sledgehammer Games' master plan to re-release some fan favourite maps as part of Elite's monthly DLC iterations. Bare in mind that this may just be some leftover unused files, so take this with a pinch of salt; but we think it'd be awesome to play on Crash again. Kinda wish that Scrapyard was on there though.