Continuity mistake: There is blood on Samantha's gloved hand when she presses the keypad, but there is no blood on the keypad after she removes her hand.
Continuity mistake: When Goat dies, after the last attempt at defibrillation is made and the others give up, you can see the supposedly "dead" Goat take a big breath just before Reaper says "He's gone". In addition, in this shot Goat's eyes are wide open when the camera is on him. When the shot goes from Reaper back to Goat, his eyes are now closed. (00:41:05)
Continuity mistake: There are lines in Dr. Carmack's message when Sarge shows it to his team that were not when he first recorded it. (00:02:20 - 00:08:20)
Continuity mistake: When the female, running after Dr. Carmack, gets her hand stuck in the door, her hand is first in a horizontal position. When it cuts the arm is suddenly in a vertical position. (00:02:00)
Continuity mistake: When the woman's arm gets cut off in Dr. Carmack's door, a lot of blood spills on the door. In the following shots the blood is gone. (00:02:05)
Continuity mistake: When the Rock is wrapping the wire around his hand at the end, the first shot of him wrapping the wire has one lap done on the top of his hand, in the next shot he has 3.
Continuity mistake: When the Sarge wraps the wire around his arm, it coils down his forearm once, like a spring, with just enough to wrap around his hand and make the spike. In the next shots of him, the wire criss-crosses the Sarge's forearm, indicating a longer wire was used during the "fighting." Also, the wire sits comfortably around his arm, when in actuality it should grip the forearm hard enough to leave marks and cut off the blood circulation to the hand, with only the ends of the wire relaxing.
Answer: This is explained by the fact that it was likely hiding, waiting to see what the RRTS would do. After all, the human test subject still had its personality, as far as is known. It would've waited to discover their weaknesses, stalking them like a predatory animal stalks its prey.