Completely undetectable by diagnostic tools - and of course you can switch it on & off by the cruise control lever so even on the test drive, the tech can not feel the power increase.
I am sure if your insurance company wanted your ECU removed & sent to a tuner to confirm its chipped they could do so, but in our 10 years in chipping cars I have never heard of it being done.
Mind you, its always best to disclose everything to your insurer (all though I am sure a lot of chipping goes un disclosed :o )