A new generation of pig buildings is emerging that are usually more open and emphasise animal welfare. However, it remains unclear whether pigs prefer spending time indoors or outdoors. This study focused on a finishing pen for 20 pigs (1.2 m²/animal) and divided into two living areas: (1) a straw-covered indoor area for sleeping and (2) a fully covered outdoor area (used for feeding, watering, exercise, and excretion). The objective was to develop a system to detect individual animals in the living areas. To this end, a pen was equipped with eight UHF RFID antennas (three inside and five outside), covering 70% of the area of the pen. Additionally, the animals were equipped with individual ear tags. The antennas were continuously read by a reader, and the software queried the reader every 4 seconds to capture data on tag detection, which was then timestamped and stored. From May 2022 to August 2023, the pigs were detected on average 87% of the time, with an equal distribution between the two living areas (indoors/outdoors). For two distinct seasons (May-June and September-October the average occupancy rate of the outdoor area by all pigs varied a lot from one day to the next (between 25% and 79%). The same conclusion was drawn at an individual level. In the future, this tool could be useful for evaluating animal welfare in the next generation of housing and studying effects of the environment, new equipment or farming practices on animal behaviour.