There's a newer method to locate PEX tubing in concrete -- ground penetrating radar (GPR). GPR is a geophysical method which was primarily used for geology and archaeology in the past, but over the past decade has become very significant in construction and industrial services.
There aren't very many GPR service providers, but most major metro areas should have at least a couple. Locating PEX with GPR isn't an entry level service, it requires high-end equipment and reasonable expertise, but with these conditions met it's a very routine task.
As an example, we recently mapped out all of the PEX tubing in a 18' x 60' area in just over half a day. The area was not at the middle of a parallel tubing run, the tube layout ran around existing machinery in a complex (and cool) pattern. I've created a gallery in my profile with photos from the job.
Extending the service, highly qualified service providers can create 3D maps of large areas and recreate as-built documentation when it has been lost (or is insufficiently precise for destructive work, as is often the case with PEX). As another example, in the past I spearheaded a project in which we mapped tens of thousands of square feet in an abandoned / unfinished building prior to remodel. The as-builts were long gone with the original owner's bankruptcy. With the piping layout, they could proceed with architecture and design.
I don't mean to solicit business with this post, but to raise awareness that this technology is available. If you have any questions about it, please check
our website or ask here, and I'll be happy to answer or try to help you find a qualified service provider in your area.