The place I work has been requiring 3 days in the office per week. They started "getting serious" a few months ago. They use badge data. If you badge at least once, it counts (on a typical day you'll use your badge at doors/elevators probably 5+ times, so there's almost zero risk you can go a full day without badging, even if people hold a few doors open for you).

If you drop below 2 days on average over some period, they send you a threatening-sounding email and you have a month to get better or you'll have a meeting with your skip level manager. What happens after that I don't know.