Yesterday my Fitbit clocked my walking steps without wearing it all day

Yesterday I had lot of walking around, up and down on two different sites some 30 km apart. While I left for the site in the morning, I had kept by #FitbitVersa for charging on my office desk. I put it on in the evening after coming back and as soon as I strapped it around my left wrist, it clocked 7000+ steps. I was not wearing it all day, my #fitbit was on my office desk all day yesterday, getting changed. How come it did clock those 7k+ steps in the evening as soon as I strapped it on? Can anyone explain this?