One of the messages from the Vigo painting is a reference to the O.J. Simpson trial. It reads, "I see the evil in the times to come. The glove won't fit."
Look at the wall behind Peter's desk. Some of things found there are funny.
Complete the indicated task to unlock the corresponding award.
The player ranks in order are as follows.
Check your messages when you get back from the Stay Puft Marshmallow level. The new person's father or friend says "You're doing adequate work out there Luke, keep it up!". This reveals that his name is Luke Hoss.
When using your PKE Meter to track Slimer, go up to Room 1206, to the right of the vase he is hiding in. The sounds coming from the door are of a man and a woman arguing (or doing something else).
Back in the firehouse after the "Shador Island" level, there are Spanish newspapers thrown all over. The headline reads "Un Cubano En El Cosmos," which means "A Cuban in Space."
When hunting Slimer, you will find room 1221, the Spider Witch's room. Looking through the Paragoogles at the lower left decoration will reveal a glowing spider.
When you are in the firehouse, go down by the containment unit. Look on one of the work desks to see a circular ghost trap that was used in the late 1990's cartoon called The Extreme Ghostbusters.
In the basement, on the far work station there is a PKE meter modeled after the one from The Real Ghostbusters.