Organization & modularity
Where a hidden pocket sits decides whether you can reach it, and which features slow a thief.
A pocket isn't hidden because the brand calls it hidden. It's hidden when someone standing behind you in a crowd cannot see it, cannot reach it, and cannot tell the bag has it.
Hidden pockets are positioned to be non-obvious: against the back panel, inside another pocket, on a shoulder strap, or at the edge of the back panel. Placement sets reach. A pocket against the back panel can't be reached while you wear the bag. One on a strap or at the edge can, and is easier to find. A pocket you can open one-handed on the move is a quick-access pocket; it is only hidden if its zipper is concealed.
Security zipper designs include concealed pulls (blending into fabric), dual-pull zippers with lockable loops, and zippers that need two hands to open. RFID-blocking pockets contain a metallic mesh meant to stop electronic skimming of chip-enabled cards and passports.

In crowded places (markets, transit, tourist sites), opportunistic theft targets visible, easy-to-reach outer pockets. Hidden placement with concealed zipper pulls makes theft slower and less likely. RFID blocking costs little and does little, and having it is not a fault; see RFID blocking.
Set the bag down, step back a few paces, and look for the hidden pocket: no visible zipper, seam, or bulge should give it away. Then check concealed zipper pulls. Are they flush with the fabric when closed? If the protection matters to you, test RFID blocking at home: put a contactless card in the pocket and hold it against your phone with an NFC card-reader app open. If the app still reads the card, the pocket doesn't block. Check lockability: paracord through dual zipper pulls plus a small carabiner makes a light lock. It stops a casual unzip, not a determined thief.

For bags used only in secure, low-risk environments (office, home, familiar neighborhood), security features add complexity without a matching benefit. If you carry minimal valuables or prefer keeping them in a money belt, hidden pockets are less important.
Key takeaways
Quick poll
In a crowd, where do your wallet and phone go?