On underground stations you will usually not see the chipcard portals/posts, as they will not be on the platforms. However, a few underground stations have got them; at Rijndam Airport Terminals 1-3 (the line M4 station), portals have been placed at the ends of the platforms, where you can transfer to the Airport Shuttleservice. As the Airport Shuttleservice is not part of the Rijndam metro network, but belongs to the airport (price included into the parking lot fares), you will have to check-out of the metro network at the airport before boarding the Airport Shuttleservice. Or check-in to the Rijndam metro network if you are transferring the other way.
At another underground station the portals have been placed on the platforms too, as the station is very close to the surface, and there is already scenery at the surface (so placing nothing "because it is out of view anyway" was not an option
) in a way that made the placement of portals here difficult. So I placed them on the platforms.