Unfortunately thats not a good enough argument if you get caught with undeclared mods. I know someone who works for an insurance company and you'll surprised what lenghths they will go to to avoid paying out on a big claim. A small one i'm sure it wont matter but some firms will go out of their way not to pay out for a write off. A common thing is for an investigator to take a quick look at your residence from the outside. If it looks clean and well kept then you're less likely to be a scumbag! That even extends to the state of your garden! No doubt this is for more extreme claims but thats just an example of how sneaky they can be. I couldnt say whether they will be able to detect an ECU remap but if I get one then I'll declare it anyway. As was stated in another thread, some firms charge alot for ecu remap, some charge a little and some charge none at all.
Imo its the same as not declaring alloy wheels, bodykits etc. The fact that its hard to detect isnt the point, its still fraud whatever kind of spin you put on it.