Suburban Intermittent Low Coolant alarm

My 2000 Chevy Suburban (Vortec 5.3 engine) occasionally flashes a trouble alarm that the coolant level is low. By occasionally, I mean maybe a total of 5 times during the last 6 months. It occurs after an
hour or more of driving and it doesn't seem to be affected by ambient temp. or engine temp. Engine temp always runs normal. Last week it came on for the first time
in several months. When the alarm comes on. I have verified that the level is low. I looked at my owner's manual and to my surprise, it suggests that the level should fall slightly when the car reaches operating temp. Yesterday, I verified that this does occur although it fell only about a half inch or so. Why should the level fall? The only answer I can think of is that the hoses expand when they are hot and pressurized. I believed that explanation more when I saw the level go back to the COLD normal when I cracked the cap and let the pressure subside. I have never added coolant in the 2+ years that I have owned it. This past June I made a 4000 mile trip and the alarm never came on. Similarly, in September I was a hurricane Rita evacuee stuck in endless
traffic jams and the alarm never came on. So, my questions are: 1. What could cause the level to occasionally and unpredictably become extra low when the engine warms up? 2. Why does the level fall at all with increasing engine temperature?
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I would check that the sensor itself is clean and free of any debris that could affect its operation. Check that the sensor is securely fastened in place and not getting some erroneous reading due to movement.
Good luck and let me know what you find out,
Blair

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