Thank you for the replies!
I am from Illinois and I was on I-294. My radar detector picked it up as a K-band radar (I have a Escort Solo S2). After reading what steagall1000 said, I believe this was the case. I did see another police officer not too far down the highway on my side of highway, but he had someone pulled over already. Maybe that first police officer was calling down the road a description of the vehicle that was speeding.
SpeedCheetahAl: Thanks for clarifying about the mobile camera vans, I wasn't aware someone was inside of them. Also thanks for the information on the different types of equipment.
On the part of the highway I was on, the highway slopes down a little then slopes back up, and on the way up the slope you cant see far ahead of you. As soon as it levels out its a long straight away and you can clearly see down the highway, and thats when the radar detector went off and I could see him off in the distance on the other side of the highway. I wasn't in a construction zone or anything, but after seeing the signs on the highway saying that its monitored by speed camera (usually only found in the construction areas) I was wondering if the police officers could sit with a radar and speed camera and just send you a ticket via the mail. Again, after reading what steagall1000 said, it seems that it may have been a police officer telling another officer down the road of a speeder.