The hiring manager wants to know the approach you took to find the vacancy. If a current or ex-employee told you about the position avoid mentioning their name, because they may have left the company on bad terms. Also the hiring manager may feel pressured into consider you over better candidates.
Find a publication which the company has featured in, and base your answer around that.
For a project manager role in the automotive industry a good answer could be:
I’ve always had a passion for emerging technologies and I’m subscribed to many automotive journals. In last month’s addition there was a story about a driverless car developed by your company. Further research into the technology lead me to your website where I came across the open vacancy.