How was I detected?

It's early 1940 and I had just received a report on a convoy heading my way. I basically parked my sub right in front of the convoy's heading so that I would be in the midst of it during its approach (there was a juicy C2 in the middle).

Anyway, I was at a deadstop, rigged for silent, when the flower corvette in front passes right by me and makes a U-turn. I thought it was just acting normally when it starts heading for me again. Before I know it, it starts pinging me and I hear the "We've been detected" warning. How did it know I was there? Do they have a passive radar or something that's always active?