Every good smuggler needs a good ship, and every good smuggling ship needs some good smuggling compartments. A typical smuggling compartment is concealed beneath a ships deck or hull plating, and consists of a simple hollowed out space created by shifting around or re-routing the components in the way, or making use of the limited deadspace present in some hull designs. Most are airtight or otherwise sealable, and can be opened by interior controls or by hidden controls somewhere outside the compartment. Most simple smuggling compartments are not actually shielded from sensors, but enterprising smugglers locate them near power conduits, plasma fuel lines, and other such sensitive equipment to disguise any sensor signatures that their cargo might put out.
More advanced version might incorporate scatterweave panels or actual shielding, but this is exceedingly rare and an expensive endeavor.
This modification is usually seen on light and medium freighters, though some pinnace, courier, and shuttle craft can use smaller versions. Fighter craft are too small to give up the space, and bulk freighters and the like are big enough that these compartments are redundant and unnecessary.
Smuggling compartments can provide up to 2 cubic meters (or .5 for smaller craft) of concealed cargo space that is hidden from cursory searches and casual scans. Thorough focused scans may reveal the presence of the compartment if a scanning crew gets close enough to them. More advanced and difficult to obtain versions using shielding, scatter weave, or particularly clever placement and concealment can be hidden from all but the most thorough, time consuming inspections.
Concealed compartments themselves are not typically illegal, but smuggling definitely is. If law enforcement find these on your ship, even if they're empty, you may get flagged for inspection every time you enter a system. Shielded or scatterweave lined compartments are highly illegal.
An import from last timeline's stock tech. This is a entry level ship modification for perfect for smugglers transporting illegal goods, but who don't want to draw unnecessary attention with a bunch of cannons or other flashy systems.