Like storage capacity, this upgrade allows gear to carry smaller gear within it. However, the gear carried in a secret compartment is concealed. It takes a DC 30 Perception check to notice the compartment when is is used, and a DC 25 Perception check to fin it with a close, hands-on examination. Equipment with a secret compartment upgrade can hold a single item one size smaller than the equipment itself. This is actual size, not weapon size. Thus, a blaster pistol (Small weapon, Diminutive object) with a secret compartment can hold a Fine-sized object (such as a comlink or frag grenade).

Source: Scum and Villainy, pp. 41-42