You have modded the engine mate, doesn't matter how.

DTUK box will increase mechanical, thermal, etc stresses on engines and certain other components (DPF for example). Not saying it will certainly cause damage but where there is more stress there will be more wear if you don't change engine mechanically.
Am sure warranty rules and regs point this out somewhere (VW being german certainly have this covered). On the plus side you have only voided engine and assosiated bits warranty (gear box, drivetrain) but not the whole car, say the chassis.
Just tried to point out that technically you have voided warranty, if you can sleep not telling VW that you had tuning box fitted then don't

Don't worry too much, could be something else completely unrelated as well (and you're not alone going over 3k5)
