Should I place a spoon in a cup of liquid before heating it in a mircowave?
This sticker is indicating caution when you heat a liquid. A microwave can induce superheated water which can be dangerous. If your water is superheated, it may not explode/boil over until an object is placed inside the water (e.g. a spoon). One way around this is to place a chop stick or object into the water while it is heating. This will prevent that nasty surprise when you make your earl grey, hot.
Now, putting metal into a microwave does no necessarily cause sparks. In fact, you can actually put tin foil in a microwave at times and not create sparks (Disclaimer!! Do not try this at home). The sparks are caused when you have very thin pieces of metal. So when tin foil does spark, it is at the very edges where tearing has caused the material to be thinner than the bulk.
Other fun things to do with a (preferably someone elses...) microwave: this and this.
From the Bosch Microwave/Speed Oven manual:
"Liquids, such as water, coffee, or tea could be overheated. They can be heated beyond boiling point without appearing to boil. For example, visible bubbling or boiling when the container is rmoved from the microwave oven is not always present. THIS COULD RESULT IN VERY HOT LIQUIDS SUDDENLY BOILING OVER WHEN THE CONTAINER IS DISTURBED OR A UTENSIL IS INSERTED INTO THE LIQUID. When heating liquids, always place a glass rod/spoon in the container. This will prevent delayed boiling.
CAUTION Metal e.g. a spoon in a glass must be kept at least 1 inch (25mm) from the oven walls and the inside of the door. Sparks could irreparably damage the glass on the inside of the door."