A hidden sensor structure for a paintball gun is to secure a loader unit on a hopper of a barrel. A loader is used to hold against the loader unit. The loader unit has arcuate slots to receive resilient stoppers therein, with studs protruding from the resilient stoppers to extend into the hopper. A sensor has an emission end, a receiving end, and a socket. The sensor is coupled to the loader unit from the bottom of the barrel, with the emission end and the receiving end inserting into two troughs formed on two sides of the load unit and the socket engaging with a fixture of the loader unit.