From Video Game Music Preservation Foundation Wiki
Investigating is the song that plays when you're looking through the casino and when you are in the sewers.
Investigating is an unofficial title. The composer should be contacted for an official title.
Déjà Vu (NES)
This song is track 6 in the NSF file.
Déjà Vu I & II: The Casebooks of Ace Harding (GBC)