ok I will give you "some" examples of how your data can be HACKED into..
GPS: (no internet or data ability).. you mark waypoint's while out in the field and go home then connect the GPS to a computer to backup your waypoints using the GPS provided software.. your now exposed as the software can easily relay the data back to the developers server when your next online - even if you tell the software not to!
Phone: phone apps can track your movements, even if you tell the app not too! The data can either live feed back to the developer's server or relay it back at a later date.... all without you knowing!!
Recently on this forum I came across a company who provides live mapping services for prospectors. They swore black and blue they dont infringe on your data or movements. So out of curiosity I read their software privacy and end user statement when I was blown away with how many loopholes the statement had which would easily legally allow the developer to spy on your data. I'm no legal professional but I'm also no fool. Just for the record, I recently held a senior management role with a FTSE top 100 company where I was often asked to review draft contract legalities for their Asia Pacific operations prior to submitting to their legal department for final approval. I enjoyed that role.
You can scan and create custom gold maps yourself, just get hold of a GPS that has custom mapping abilities, most good Garmin units have this. Save all your tracks, waypoints ect on a SD card using your GPS, then backup the card on your computer. Delete all waypoints ect on the GPS before connecting it to any computer. When you want the waypoints on the GPS again, just install the SD card with them back into the GPS and your good to go.
Yes there is a lot of data they would have to sift through, but remember the developers are smart, they would simply develop an app and have it do all the sifting for them using algorithms!
You got to remember, what all these developers are doing is nothing more than what "you" can do right now for free!!! The maps are out there, just scan them and create custom maps for use on your GPS and off ya go.
Good luck!