I think the OP was saying what COULD have happened.
How in the world can 2 subs strike each other? Don't they have that SONAR thingy that is all the rage on underwater vehicles these days?
Sure--boats have sonar. Active and passive. And if you want to broadcast your presence to everyone several times you return detection radius, go ahead and use that active sonar.
Passive, OTOH, is listening for sounds both in and around the boat, and behind the boat, using what's called the "tail" or towed array sonar. All it does is listen for sounds a boat, surface ship, or even low-flying airplane might make, such as machinery, screw cavitation, or splash noise. But when a boat doesn't WANT to be found, it can be a daunting task at best to find a boat.
Imagine you and your adversary are in a dark room with all sorts of surrounding ambient noise, and you try to focus on specific sounds and/or frequency noises. Sure, you could use your flashlight (i.e., active sonar), but if you didn't light up your target immediately, the other guy would know where you were immediately without you ever being aware of them being out there. That's when you go from hunter to hunted.
Collisions between American and Russian boats have happened. It's actually surprising that there weren't MORE of them. That being said, this "senior Navy source" was probably some dipshit E-3 (equivalent) talking shit in a bar in London somewhere. COULD the boats have been lost? Sure--however it is very unlikely unless the collision happened at high speeds. Could there have been a "radioactive leak"? Even more remote.