I had a company do this to me once - a minicomputer company in the 80's. A friend and I had written network printer management software for Prime minicomputers because Prime's "spooler" had lots of flaws. Some Prime engineers called up, wanted to talk about acquiring the software. Of course we were ecstatic. They flew in a couple of engineer types for a couple of days, we talked about various aspects of the product, then they wanted to take the code with them to review it with others. I wouldn't give them a digital copy, only a paper copy. They weren't crazy about that, but took it with the agreement they'd return it in 30 days. I never heard back from them, they didn't return it in 30 days, I had to badger them to get it back, they eventually did return it, and about 6 months later, came out with a new spooler product that incorporated many of the ideas in our product. I can't say they copied code, but they did mislead us IMO. In hindsight, I would have just given them paper copies of parts of the code - nothing complete, not even a complete function unless it was rather trivial and something that anyone could code in a day.
Good luck!