DaveB of the MTC forum piqued my curiosity when he asked about it and I just had to check with the Silhouette folks to find out. This is what they said:
The noted “teardrop” is a sensor for when the “Child Safety” feature is enabled. The Child Safety feature may be accessed by loading your mat and then pressing the LEFT and ENTER buttons at the same time. This will bring you to an options sub-menu. Here you will find a “Child Safety” menu. If you select to “Enable” then the Cameo will immediately cease operation any time the lid is lifted during its operation. Please let us know if you have any further questions.
The Silhouette Support Team
A child safety feature on my Cameo? Who knew?! Yet another reason to love my cutter. :-)