Most anyone able to charge market rates might be more motivated to reveal they've been contracted to do this than to actually do that.
Someone who has a hard time getting market rates might not be as qualified to fix the software as they think they are.
There are places in the world were good computer / science education doesn't necessarily mean being able to demand "market rates" comparable to the West. I can see some programmer n Russia or Romania being tempted by a side-deal like this. Especially if presented with a Western "market rate".
Many programmers in Russia and some eastern european countries probably wouldn't even see that as a "side deal"
I haven't been back there in many years now, and tried to give the benefit of the doubt ;-)