Finding the Ghul Matron in Shadow of Mordor

First of all, Ghul Mounds are not a creature, but instead a terrain anomaly caused (lorewise) by Ghuls. Think of mole holes, but instead of having a mole come out, you have 200 Ghuls jumping at you. Here's one:

There are only two ways to find them, like you said, one of the methods is in caves. The second one is finding those mounds around non-stronghold areas. One important thing to keep in mind is that they only spawn at night. The mounds (and by extension the Ghuls) don't have a fixed spawn, so you may have to run around for a bit (and by using the wait command in the Map to respawn).

Once you found a bunch of Ghuls you have to fight them, but don't kill the fleeying Ghuls (The ones with the 3 claws indicator over their heads). They will run, which triggers the spawn of Ghul Spitters. Repeat the process, and around the 3rd/4th wave a Ghul Matron spawns