Continuity mistake: When Santana begins to "spit noise" to intimidate Riddick, the camera moves around him and we can see the station and some mercs in the background, one of which is Falco and he is alone, but then the shot changes and we can see Vargas next to Falco talking to him. The shot changes again to Santana and when he is lifting the box, we can see Vargas approaching Falco in the background.
Continuity mistake: When Santana is trying to open the locker, after he enters the third "click", he talks with Johns and looks at him, putting his back to the locker. In one shot Santana is near the locker, but in the long shot, he is further from it.
Continuity mistake: When Santana is going to open the bloody written locker, he takes the key off his neck, but when he puts the key in the keyhole, it's around his neck again. Some moments later, when he is talking with Johns, the key is off his neck again.
Answer: Earlier in the film when he arrived at the base, he could see the monsters from a distance when the rain came in. That's how he knew.
Gavin Jackson