Backup
Before they made their way through the door, they could hear some men approach the house from the outside. They lifted one of the officers up and he was able to wave at them and get their attention through the archway. He told them to be quiet and to wait for them outside.
Now, with backup waiting for them outside, the three of them snuck past the creature silently and went through the mysterious door. To their amazement, the door led to a tunnel that went straight down into the darkness. They looked at each other and kept moving forward into the tunnel.