If you only topped off the radiator, you probably are still low.
-take off the radiator cap (with the car COLD!)
-fire it up
-once it starts to get warm, the thermostat should open, and the coolant level should go down.
-continue to top it off until it will not take anymore
-replace the cap and make sure your overflow is at the correct level
Being low on coolant should trigger your low coolant light, and the buzzer. Also, it can screw with your idle, since the sensor may not have adequate contact with warm coolant.
The idle on my Miata was high, and I could not figure out why. After I topped off the coolant, it settled down, and is now perfect.
Damn fuel injection
