It's another feature in AoE3 and BFME2. Basically, there are random (or specific, if skirmish) "dens" where some treasure is hid. If a player defeats the guardians (Gaia owned defenders), then they get the treasure. The treasure varied a lot and its value was based on how many / how strong its defenders were. Some examples are: money, special tech (usually not available any other way), special mercenary soldier, tamed wild animal, etc. Nothing game changing, but it can add some variety.