Head to the cemetery by the Demon Door . The key is opposite to the entrance .
If you go to the monorail station and start going towards the lake, you will see a wooden fence by the road . Jump over it and run forward along the road. At its end you will find the key .
Now return to the main route and head towards the lake. Turn right into the forest beside the road signs pointing Bowerstone Market. This way you will reach the Dankwater Cavern, opposite to a small lake . Jump inside and swim left. The key is waiting for you there .
Opposite to the monorail station, you will find rails leading to the mine with a boarded up entrance . Get rid of the planks with a couple sword slashes and enter the cavern. At its end you will find the key.
To get the next one, you have to complete "Reaver's Unmentionables". That way you will reach a secret room inside Reaver's mansion . The silver key is floating behind the bed .
After completing "Hobnobbing with Hobbes", rest for a couple days and afterwards return to the cave . Stick to the left side and you will reach an underground lake .
Enter it and swim maximally to the left . The narrow path full of Hobbes will lead you to the key .
The second silver key can be found in the same cave, but on the right end of the lake. You will also find a small hill there  and the key atop .