Something confuses me about the police, fire department and rescue. Something deep down urges me to question their methods. There seems to be a vast number of different levels of emergency in the world. What causes cops to drive normally and then burst into a 100 MPH chase with full sirens is usually no mystery. Generally there is a bank robber or jewel thief "at large" and this calls for "the fuzz" to go into a full out alert. That doesn't confuse me one bit. What confuses me is all the stages between those two that don't seem to be as important. Here are all the various stages as I see them:

No Siren, No Speeding
This is the emergency vehicle driver's default setting. Generally the police can be seen prowling around the city you live in, looking for trouble. They're the ones in the blue uniforms shooting you accusatory looks when you're pretty sure that you haven't done anything wrong. The only thing worse than this type of behavior is when you drive past one of those regular people who has taken it upon themselves to be a "citizen on patrol". They're the ones who gestures with their hands for you to slow down. I just encountered one of these people the other day driving 30 MPH in a 25 MPH zone. Apparently the thought that someone as reckless as myself was on the road concerned her as a parent.
My "Emergency" Guess: There's probably someone being mugged in the near vicinity.

Ambulance on the Highway
This one confuses me. I'm driving down the highway then all of a sudden traffic slows down and I realize that there's an ambulance up ahead just cruising ever so slowly down the highway. This raises an important question: If this emergency is so dire that it requires the hospital of another city then why does the ambulance only travel at 35 MPH on the highway? Do they get paid by the hour? Are they actually milking the clock? This is more of an annoyance than anything else, mostly because I always feel like I would be doing something wrong if I passed an ambulance.
My "Emergency" Guess: The guy in the back has just died and the driver wants to take a Sunday Drive.

Speeding, No Noise
This is probably just speeding. There's probably nothing happening that is an emergency in the least, they're just driving fast. As anyone who has played Police Quest 1 knows this is known as a "Code Two" and it can easily be achieved by pressing F8. Be careful though, if you drive through a red light without going to a "Code Two" you'll lose a point!
My "Emergency" Guess: Yellow means, speed up before it turns red!

Flashing Lights, Speeding, No Noise
Emergency vehicles are usually being called to save lives yet nothing I can imagine is more of a danger to lives than a really quiet police car speeding through a red light. Wait, maybe there's something. Yeah, I think that a really quiet FIRE ENGINE speeding through a red light may be more dangerous than that. Even more dangerous may be when I'm driving through a green light and there's a huge church on my right so I can't see the street and as I turn left I happen to catch a glimpse of a silent fire engine coming at me at 80 MPH. Then I would have to speed up incredibly quickly and actually drive up onto the sidewalk to avoid dying. Then I would thank god that I was driving a really small car. I would also thank god that I somehow managed to avoid a telephone pole and a concrete wall. Then I would do a quick check to see if I soiled my pants. This is all completely hypothetical of course. Nothing like that could ever happen.
My "Emergency" Guess: The driver forgot to turn on the siren.

Short Bursts of Sound with Flashing Lights
This one I saw just the other day. I was sitting on my porch and I saw a cop speeding by. Apparently there was something semi-emergency like happening and he was just putting his siren on for short little times. The result is a rather comical police car, burping and hiccuping down the street. I can't see what could warrant this type of behavior. It seems like it would just be a lot easier to leave the sirens on rather than switching them on and off. Maybe this is what police officers do to pass the time when they get bored. Maybe they would be better served to take up a hobby like giving out tickets or oil painting.
My "Emergency" Guess: Driver is about to die of boredom.

Full Out Sirens
At last, a real emergency. Aside from when a vehicle is parked or driving normally this is the only easily acceptable state. It is clear what is happening: an emergency. It is also clear what should be done by other drivers (at least other drivers who'd had driver's ed).
My "Emergency" Guess: A cat burglar or some evil ninjas are robbing a bank.

*Chefelf Supplemental*
I just realized I got through all of this without making a joke about police officers and their above average desire for morning pastries, so here it goes: Maybe it's because they're too busy thinking about donuts! Ha Ha Ha!!! Cops like to eat donuts! Remember to get a jelly donut for the chief because I think I smell bacon!