Depending on how long you were not wearing it, those could be your BMR (basal metabolic rate) calories which you are always burning just by being alive. Fitbit always counts them even when you are not wearing tracker.
See How does Fitbit estimate how many calories I've burned?
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Hello @JoanneH14, it's good to see you around in the forums for the first time, welcome.
Thank you for your inquiry about the calories, just as @JohnnyRow explained earlier, the calories burned while you're not wearing the device will correspond to the basal metabolic rate (BMR), which is the rate at which you burn calories at rest just to maintain vital body functions like breathing, heartbeat, and brain activity. This is estimated based on the physical data you entered when you set up your account: sex, age, height, and weight. Make sure that all the information in your account settings is correct.
If you do feel that the amount of calories burned is too high based on your personal setting, please try to restart your watch following these steps, and check if you continue to experience any trouble.
Let me know if you have further questions.