And according to other articles I've read, most listening is passive, and with two quiet boats, there wouldn't be anything to listen to.
all ships make noise as they operate, and reactors are relatively noisy. They can't be turned off like the electric motors of a conventional submarine, and their cooling motors need to run constantly. Now, they do everything possible to reduce their sonic signature underwater, but there's a limit to any technology.
Yes, submarines avoid using active sonar while on patrol. They use passive sonar. Attack boats, also called Hunter-Killer or HK subs, are used primarily as anti-ship weapons delivery platforms. They use surface-to-surface missles and torpedos to attack shipping, especially other submarines. Since their assigned prey includes submarines, including ballistic missle boats or boomers, it's my understanding that they need to be very quiet. The boomers are assumed to have the best sound gear available, so the HK boats that oppose them are stealthy.
From what little I know about submarine and anti-submarine warefare, any HK boat that stumbles upon another sub will follow it to study its operations and perform simulated attacks. The sonic signatures will be recorded for the navy's intel folks to study, and they'll often have signatures for known and suspected submarines on file.
The ocean is a big place, so the chances of two submarines from different navies attempting to occupy the same space are quite remote. Therefore, I would expect a much higher probability of collision between two boats that were shadowing eachother than just "passing in the night".
Now, I don't have any idea what missions these two subs were on-maybe their proximity was not randon chance. One never knows.