My guess is that they noticed something was not quite right when inspecting the code and then tested their hypothesis manually to see if it was actually a real

My guess is that they noticed something was not quite right when inspecting the code and then tested their hypothesis manually to see if it was actually a real bug. Something similar happened to me when I was working on some code that should clear the state and, by inspection, I noticed some field was not properly cleared under specific circumstances. Then I tested the hypothesis and indeed found a bug. Sometimes intuition and guessing is what it takes.