To record that a participant did not show up for their commitment, simply remove that participant assignment on the SignUp so it does not get included in the participant's summary of hours. Please see: How can the organizer delete a participant assignment?
If you want to track hours for drop-ins, just add an assignment to the date after the fact. You may add a manual assignment for a participant to a past date to account for drop-in hours. Please see: Can the Organizer add an Assignment for a Participant? If you do not have any spots already planned on a past date, click 
on the upper-right of the calendar instead of clicking on the date. ( You will need to change the date, which defaults to today's date)
Note: SignUp.com will automatically track service hours for ToDo items (jobs, shifts, tasks) with both a Start and End Time. Items with just a Start Time are given the default value of one hour.