When you get to the end of a section (for example, going down the stairs to hunt down the Grey Lady ghost, just after they brief you on the P.K.E. goggles), put on your P.K.E goggles and walk through the door or off the staircase. After the other Ghostbusters are finished speaking, you will notice you cannot see your character. You can only see through their eyes, and you can clearly see the trails of ectoplasm (the little colorful clouds of ghost goo) without the goggles on. To exit, return to the firehouse or wait until you get to the next part of the level.
In Ghostbusters HQ, there's a table upstairs with a phone. Interact with the message machine to the left of it to listen to a variety of voicemail. There is new voicemail each time you go to Ghostbusters HQ.
Unlock the following options at the "Options" menu by performing the corresponding tasks:
OPTION - HOW TO UNLOCK Equipment Strength Upgrade - Collect all 103 art illustrations for Tobin's Spirit Guide. Faster Health Recovery - Collect 50% of the art illustrations for Tobin's Spirit Guide. Gozerian Rookie Outfit - Complete the game 100% under the Gozerian difficulty. This outfit will allow you to be immune to sliming. Increased Scanning Speed - Scan 50% of the ghosts and scannable objects for Tobin's Spirit Guide. Invulnerability - Complete the game with all 103 art illustrations collected and scan all 103 of the ghosts and scannable objects for Tobin's Spirit Guide. No Equipment Overheat - Scan all 103 the ghosts or scannable objects for Tobin's Spirit Guide.