I was trying to position the sticker (for the car parking, just a label) on the inside of the windshield when it slipped and disappeared into the gap between the windshield and the dashboard panel. It's about half the size of a palm and it slipped before I removed the film to expose the sticky side. There is no way to retrieve it without taking the car in to the shop and having parts removed. Should I worry and try to retrieve it? Any possible issue if I just leave it buried there?

Welcome to NICO Club! A sticker or square paper won't cause any issues with the stuff under the dash. The only worry I'd have is getting a ticket for parking! As long as you don't need the sticker, then you don't need to remove it in my humble opinion. To get the sticker, you may have to remove the dash, but you could spend some time with a flashlight looking up from underneath to see if you can grab it somehow. I would not pay a shop for this work. I'd leave it if I couldn't get it myself.

You don't have to worry too much about the sticker. if it's made of paper it will degrade in the long run. moreover, getting it done at a shop will cost more money than its worth. just leave it there if it doesn't do anything that bothers you.

3x to that..........

Check your cabin air filter, it probably get sucked onto there...I cleaned mine today for first time and no lie, looks like it hasn't been cleaned since 1998