Visible crew/equipment: A boom mike is reflected in the car window, when Alex pushes Hugo's car into the lake.
Plot hole: When Juliet and David spend some money, they are seen watching a tape of the footage they filmed with a new camcorder. When Alex is in the bath, we see him with the camcorder (it is not a reflection in the mirror) - where did the second one come from?
Visible crew/equipment: Alex and Juliet are cavorting around filming themselves with their new camcorder. In the shot of Alex wearing a black plastic hat and shades you get a quick glimpse of a third person standing very still in the background also filming with a camcorder. It appears to be a tallish fair-haired man wearing a long-sleeved white shirt. Alex and Juliet are supposedly the only two people present, with Juliet holding their camera, so presumably it's a cameraman taking some extra footage from a different angle.
Answer: Even with head, hands and feet removed, a lot about a person's identity can be determined from body scars, tattoos, body tissue and blood samples, etc. Sex, age, height, weight, body-fat content, race, hair color, and pre-death physical health can all be determined rather easily through traditional means, even given only a torso for examination. Once investigators have a general idea of identity, they can compare their findings to a missing persons database and narrow it down further to likely matches. Then they can request DNA samples from the families of likely missing persons and compare it to the DNA of the corpse. Of course, if the corpse was never reported as missing, that would bring the investigation to a dead-end.
Charles Austin Miller