If you'd like to be notified if an employee punches in using "No Shift" rather than an assigned shift, you can the Punch In Alert (No Shift).
First you will want to go to Settings --> Alerts & Reminders.
Once on the Alerts & Reminder page, you'll want to select +Add a New Alert. You'll then be asked to enter a name for the (which will be included in the alert message) and select the Type of alert you want to create. In this case, you'll want to select Punch In Alert (No Shift) as shown below:
You will need to select which user you'd like the alert to be sent to and using which method. Lastly, you will need to assign which employees you'd like the alert to trigger for. Once done, hit save!