Next time, try making some "huffing" sounds at them. I've had them come right up to me to check me out. Here's a YouTube video that has the sound they make. Bit of an odd video; not sure what the animation is all about but the sound is accurate.
Yeah I heard them make that sound a couple of times. I thought they were snorting. Never heard of Giant otters killing caimans. My family and I caught some brief, distant glimpses of a family of them during a guided day trip in the Peruvian Amazon ten years ago.