Orc Mine

The two orcs at the mine entrance disappear inside after an hour, with the sun finally setting behind the mountains to the west. After waiting another 10-15 minutes, the party can make their way to the entrance.
As they approach, they can see what the orcs have been working on. The entrance to the mine is boarded up, with broken tools and mining detritus scattered in front of it. A DC10 perception check coupled with what you witnessed earlier proves they were trying to mask the entrance to look deserted.
If anyone wishes to check for tracks or evidence of the gnomes, let them roll a survival check. With a roll better than 18 they notice a scrap of the gnomes' colorful clothing caught under a pile of scrap near the entrance. With a 15 or better they see evidence of at least 8 orcs passing through the entrance.
There is also evidence that the orcs have been using this entrance for several days at least. The characters can make perception checks to determine if there is any activity coming from the mine, but they don't notice anything. The party can enter if they wish. The mines are dark and unlit, and look as though they have barely been used in years, though there are a number of fresh tracks leading into the mine.
There is a sketch above with a general sense of the mines system. The box represents the entrance, and the lines just show the general track of the tunnels. They cut into the hillside in something of a maze, following the mineral veins. To use this as a map, assume there are small cutouts and chambers throughout the mine, as desired.
The orcs have taken the gnomes to their base of operations near the center of the sketch above. They will post an orc or two at the nearest intersection in any direction, with at least 4 orcs remaining with the prisoners in a small 20x20 chamber basically at the center of this drawing.
If the party explore the mines using a light source, the orcs will notice their approach before they are spotted, and wait in an alcove off the tunnel for the party to pass before jumping out with a yell that will bring allies from the other direction. The party can also encounter random monsters while exploring. I made this sketch vague to allow the mine to be as large or as small as desired, giving plenty of opportunity for underground encounters.
If you are looking for a more adventurous encounter, have the orcs notice the movement of the party and start out after them. It will turn into a giant cat and mouse chase around the mine, with the orcs moving as the players move. They could all make stealth checks against each other's passive perception to note movement of their opponents. Bad rolls can pick up on false directional information. I'd say if four party members rolled with some success and some failure, give the players conflicting directional information and let them decide what to believe. The catch is that you can't say "you rolled and 18 and hear this, while you rolled a 7 and heard this," or they'll know which one is right. Just say "you hear a noise, but disagree over where it came from."