This does sound like the Error Checking feature. In Excel 2003 and earlier
the color is green. In 2007, the color is customizable (so it might be blue).
To test, try turning off Error Checking and see if the symbols disappear.
Click on the Office Button
Click on the "Excel Options" button
Choose Formulas on the left
Uncheck "Enable background error checking"
Click OK
HTH,
Elkar