To mark all scavenging locations you will need to build the survey crew in each of the four strongholds. To find the survey crew parts walk up to the outline of the project in each stronghold and it will give a prompt to mark the locations for how to get the parts. From there retrieve the parts and build the survey crews in each stronghold. You will now have all scavenging areas marked on your map for the territory you built the survey crew in. Please note that there are scavenging location in the dunes (buzzard territory) and these will be marked upon completing the survey crew project for the Friah/Gastown stronghold. This will also help with finding all the historic pieces as well.