What should I document before and after detection?

Document from the moment you notice the problem. Take photos of damp spots, stains, warm areas, and any visible damage, with dates. Take photos of your meter readings with the time. Save your water bills that show the jump. Keep a short log: date, what you noticed, what you did, and who you called.
During detection, you get a written findings report that records where the leak sits, which line it is on, and what repair options fit. Keep that report with your home records. It is a clear, factual record of what was found, and it helps your insurer see the location and cause.
After the repair, keep the invoice, the repair notes, and photos of the finished work. If a mold specialist or another trade does follow-up work, keep those records too. A complete file makes every later conversation easier.