Unfortunately, this happens more often than I'd like to admit and is a serious problem and cause for concern. Luckily there are things you can do to put an end to code smuggling and with luck sometimes you can track where your stolen code is being used. Here is what to do:
You can tag your source code with certain keywords i.e. sourcesource or characters i.e. ##$$, so you can track them down in the future via internal searches, Google, etc. The true reality however is that it is hard to prevent code smuggling without having data leakage prevention controls on each workstation and server. Installing such controls now may not help you with past/existing issues but it can help protect you from future mishaps.
I would recommend reading this chapter excerpt SearchSoftwareQuality promoted, How to make your software tamperproof. Inside you will learn about watermarking and tamperproofing, which is basically checking to make sure all of your coding is in solid, good-working order and then adding characters to personalize it. This makes it very recognizable to the original coder and would confuse others trying to use it of what the function of each line of code is.
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