"....they used flexible rubber water pipes, like washing machine hoses, not the usual copper."

Are you sure they were rubber, and not plastic? Flexible plastic piping is now used extensively in domestic plumbing and heating installations. It's easy to install, uses push-fit connectors, and most importantly it is cheaper than copper. It will last a very long time indeed, and will not corrode. The plastic piping comes in coils, and is coloured grey or white.

If your installers really used rubber piping you should get them back to do the job again - rubber piping isn't suitable for concealed use, and will not have a long life.

No More Nails?

I think you'll find that a very sensitive metal/wood detector will register water filled plastic pipes. Use it on the wood setting and it will have to be adjusted VERY carefully and kept a constant distance from the wall. The change will be very slight but it is possible. I have found plastic central heating pipes under the floor in this way. Good luck.

I would consider getting them back to do it.