If you take your s550 into a Dealer for an oil change, and they see you have an nGauge and CAI - do they care? Do they void your warranty or make a note in their system that you have tuned / modified the car?
oh the internet of misinformation... They can't just void your warranty.... They can however notate that is has been tuned, and use that to deny a repair in the future OF AN AFFECTED component failure.
A dealer doesn't have the authority to void a warranty, Ford does and even then they can't just VOID the warranty. They can only void the area in which they prove you have caused damage in which would cause any issues in that area.
A dealer can DENY working on your car that's it. It's up to Ford to prove that any failure that is covered under warranty was caused by your negligence.
According to federal law the factory can deny a repair if any modification ''may have contributed'' to a failure for example, a tune may have contributed to engine failure and the factory almost always will void the drivetrain warranty. They can't deny a bad window motor if you have a tune or other mods though. Some dealers do refuse to work on modded cars.