Ignorance is no defence in the eyes of the law.
If you run into the back of me and they just maybe decide to check the ECU they might find it, invalidate your insurance and then mine!
True, they will only find the remap if they look hard for it - but do you want to take that risk?
It never cease to amaze me what people hope to get away with to save a few quid without thinking of the ramifications.
Incidentally my 1.9TDi Pd 130, remapped to 175bhp, cost £560 fully comp (mods declared!) with Noel Dazely - and I'm 24 with two previous accidents.