Upgrades (Upgrades#Universal Upgrades)

Like storage capacity (see below), 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 it is used, and a DC 25 Perception check to find 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; see Equipment Sizes.)

Thus, a blaster pistol (Small weapon, Diminutive object) with a secret compartment can hold a Fine-sized object (such as a com link or frag grenade).