Invisable Man PC
You can unlock a Cheat called 'Invisible Man' then enable it in the secrets menu! This as the name suggests turns Shadowman invisible only the masks and eyes glow effects and weapons will still be visible. Enemies will still be able to see shadowman.
N64 and Dreamcast Location:[]
Go to London Underground via Jack The Ripper. Go to the middle stall in the women's bathroom and the "Secret Activated" message will appear.
PC and Remastered Location:[]
Warp to Asylum: Playrooms and go to where the playroom cells are found (with a pool table and a playpen). Enter the middle empty cell.
Invisable Man N64