OK, shame on me twice.
My expertise is in a narrow field that found me rather than I found it. But it pays the bills. In my field I deal with large complex systems.
Not once, but twice, an (ex) friend of mine with a small company asked my to bid a project in my field. I met with him, broke down his requirements, and created an implementation plan and preliminary design – tasks that in my regular contracts I am paid to do, but as a favor did for free. I quoted a small number to implement with no back charges for analysis performed. I later found out from another person he used my bid, word for word, to get other bids for the work.
How should I have protected myself?
I did learn something, though. At a Christmas party, a VP-level person was asking about a problem in my field and I responded with next step. He told me I did not understand his problem and I just said OK.
How do you engage in opportunities without giving away work for free?