Pin&Play has enabled a new type of surface-based physical user interface, characterised by dynamic arrangement of interface objects on a surface area. Previous work has shown that this affords rapid re-arrangement of the spatial layout of interface objects, for example in adaptation to user preferences, but the Pin&Play system did not support tracking of object locations on the surface. In this paper, we investigate and compare two practical location techniques for interactive surfaces that are based on external sensing: detection of surface events using load sensors, and camera-based detection using object beacons.