Question: Why did Kurt Russell sound the alarm when the thing was attacking the dogs? He was nowhere near the area and could not see what was going on. Also before he pulled the alarm, the sound he would hear was too faint to think something was wrong.
Chosen answer: He sensed something was wrong and wanted as much help from the others as possible.
Answer: Having investigated the Norwegian camp, he could have conceived that the use of fire would be helpful. As such, he would need fire extinguishers after combating whatever the thing might be with the flamethrowers. It could have been foresight on his part that by ringing the fire alarms, somebody was more likely to bring fire extinguishers with them, thus allowing better control of the fire.
Question: How could the murderers possibly have known which swimwear Arlena would be wearing and have exactly the same costume?
Answer: They didn't. They dragged her into a small enclave and stripped off her bathing suit. The wife put it on and pretended to be Arlena sunning herself.
Answer: Remember that he'd just spent the whole day investigating how something mysterious and horrible destroyed the Norwegian camp, so he's already in a spooked state of mind. Hearing the dogs screaming at night is already unusual on its own, and also reminds him how this whole episode all started with a crazed Norwegian trying to kill a dog. Deep down he knows whatever happened to the Norwegians is now starting at their own camp.
TonyPH
I 100% agree.