The "check coolant level" message

Sensor in the fill tank is probably bad .
LOW COOLANT
The IPC illuminates the LOW COOLANT indicator in the message center when the IPC detects a low coolant level condition from the PCM. The IPC receives a class 2 message from the PCM indicating the coolant level.
Coolant Level Control (If Equipped)
The engine cooling system contains an engine coolant level switch to alert the driver in the event of a coolant loss. The powertrain control module (PCM) sends out a coolant loss signal over the coolant level switch signal circuit. When the engine coolant level switch reads a low coolant level in the fill tank, the switch opens. The message center receives its power from engine wiring harness junction block on the battery positive voltage circuit. Ground is provided by the ground circuits via the body wiring harness junction block and the engine wiring harness junction block. The cluster receives the class 2 message from the PCM indicating Low Coolant and displays the LOW COOLANT LEVEL message on the driver information center (DIC).

Is the fill tank full ?

I believe your problem may be your low coolant sensor(located in the overflow res.), they tend to get gummed up and quit functioning. Also before you chase that, be 100% sure your coolant level is actually full.

If the warmed up temperature is OK, probably not an airlock in the system. As for "check cooling level", this is shown when the integral level sensor in the reservoir tank is either not functioning or you have a bad electrical collection. This integral level sensor is not replaceable - you will need to replace the tank.

I don't believe this is a reset issue. The sensor is in the overflow tank. Make sure the level is at or just a bit above the level indicator. The sensor may be bad or blocked, but I think this is where your issue is. The sensor shows on the side of the overflow tank.

A "check coolant" message is generated when a coolant sensor (the Passat has more than one, I believe) returns an abnormal reading. That can occur when there's no coolant circulating (eminent engine seizure) or if the sensor is stuck in an air pocket.
A bubbling/gurgling sound suggests that your cooling system has some air in it, and that might be generating the error messages. If you have just flushed your system, it's possible that someone didn't take the time to purge the system of all air as new coolant was introduced. However, if the system is more or less completely full of coolant, these should eventually work themselves out. Keep an eye on the coolant reservoir tank and keep topping it off if you see the level going down.
A second possibility is that one of your coolant sensors is going bad. I just had to replace one on a 2002 Passat with only 51,000 miles on it. A bad sensor will also return odd readings that trigger warning lights.
A third, much more expensive, possibility is that you are developing a weak spot in one of your head gaskets (I'm assuming you have a Passat with a V6; the 4-cylinder engine will have only 1 head gasket). In that situation, hot exhaust gases may be leaking into your coolant, causing localized hot spots and possibly triggering the error message that you've been receiving.

There is a level sensor inside the coolant recovery tank, it may be defective, a fairly common issue.

If you are getting the low coolant message you are looking for the coolant level sensor not the coolant temp sensor. I do believe the sensor is in the jug, is it filled to the full line?

The coolant sensor and the coolant level sensor are two different sensors. Your message center is showing (coolant sensor bad),not level sensor bad.
The level sensor only monitors the amount of coolant in the container
The coolant sensor senses the degrees of the coolant, which turns on the electric cooling fans. Change the coolant sensor, and the problem should go away.

Try to tap the reseviour when the light comes on. You may have a sticky sensor inside the reseviour. If that fixes it, awesome. If not, you might need either a new sensor, if it can be pulled out, or a new reseviour.

change sensor in water tank