The party takes time to examine the doors. The first door, which the patrols of kobolds used to come in and out, looks well-maintained. The ground in front of it is barren from frequent passage.
The second door, which the last kobold tried to escape into, looks dirty on the surface, as if it hasn't been opened in awhile. Grass grows underneath the door. You also wonder why the kobolds didn't use it to switch patrols.