Foil in Microwave - No sparks?

Today at work I had a container of leftover spaghetti from Fazoli's for lunch. I removed the plastic top and set the microwave for two minutes. I engaged in some teacher chit chat while waiting. Beep. Lunchtime. I went to pick up the container out of the microwave and yelped in pain. Yes, I failed to notice that the bottom part wasn't plastic but foil. My coworker was standing nearby and laughed at me. However, I learned something today. Just because it looks like foil doesn't mean that it is. There were no sparks. She told me that many companies are now making these "foil" appearing containers out of other materials. Is she crazy or has someone else heard of this?? My Fazoli's leftovers were delicious and I didn't ruin the staff microwave! :)