Is the game installed on a secondary drive or partition, or on a removable drive?
If so, see:
https://support.steampowered.com/kb_...8379-RYIP-2998

The games installed via Steam should be installed into the default location:
C:\Program Files (x86)\Steam\Steamapps\Common
If it is installed outside of this location, then Steam has a chance to 'forget' the game is installed.