they can detect an ECU remap if
they look. Most times they don't. However if they notice other mods, like exhaust, induction kits and smaller pulleys (on the S) then it might prompt them to look at the ECU as well.
Also it depends what fails...
If you're (standard) brakes fail for some reason, then there's no way an Induction Kit could have caused it. It should therefore be covered by warranty. But if they can prove that the modifictaion caused the fault, then warranty won't fix it