The COVID-19 pandemic has accelerated the work-from-home trend, with significant variations in location, industry and firm size. These shifts have significantly affected economies and societies but traditional data cannot be easily tracked. This paper presents a method for measuring office attendance and examines its trends and characteristics. To this end, we first introduced the working-at-office ratio, which is the percentage of people going to an office compared to the pre-COVID-19 period, considering 74 office submarkets in six major Japanese cities. We captured mobility trends in office buildings using rich Global Positioning System-based mobile location data combined with specific office location data. Subsequently, we demonstrated the effectiveness of the proposed indicators by comparing them with other mobility data and office attendance indicators. Finally, we examined the relationships between the working-at-office ratio and the characteristics of the office buildings and tenants in each submarket. The findings indicated that factors, such as the proportion of large buildings, concentration of specific sectors, and tenant size, were significantly related to office attendance, with these relationships evolving over the duration of the pandemic. Our approach provides real-time, granular insights into office attendance trends, which are crucial for anticipating future work paradigms.