I'm not a structural engineer, but I don't think the presence of the PEX will significantly weaken the slab.
If you want to find the PEX, let the slab go completely cold, then crank up the heat and use an infrared thermometer or an infrared camera (I have a cool one from FLIR that plugs onto an iPhone). You can tell where the slab is by the warmer spots in the slab. It won't be exact, but you should be able to see the spacing, and get your anchor bolts between the tubing.
