I imagine the pred would have fired the speargun from a perched position up in a tree about 10-15 yards away, & considering how advanced their technology is, I doubt the weapon would use gunpowder. A speargun is capable of great discretion, & is fairly quiet while firing, so the actual shot wouldn't have been so noticeable, as there would have been little muzzle flash or sound; there would also have been no bone fragments if it was only flesh which was skewered onto the speargun round.
A speargun is somewhat like a cross between a sniper rifle & a crossbow in case you didn't know, it fires small wishbone-shaped spears that are sharp enough to cut through flesh & bone. The way you're describing it, it sounds as if you percieve the speargun as any other traditional firearm, much like the ones we use today. It is infact a vastly different weapon.
I think you misunderstood the abbatoir scene in Predator 2. Keyes' team were wearing suits that cancelled out their body heat, effectively making them invisible to the predator's thermal vision mode. Little did they know it was capable of seeing in other spectrums too. The "dust" they were dispersing into the air was a special chemical that would linger in the air & adhere to the creatures' body if touched, making him easier to spot if he was using his cloak. Harrigan wasn't intended to be part of their plan, & as a result, wasn't tooled up with any of the equipment which was used to cancel out the body heat of the capture team, meaning that the predator could clearly see him in thermal vision.