Always fill out a PS Form 3971 in duplicate when you miss work, even when you have a confirmation number, upon your return to work. This is importat in case you dont get the leave you requested and need to file a grievance later.
When leaving early dont leave a PS Form on a desk, give it to a supervisor, in duplicate, have them sign your copy, otherwise you could be charged AWOL.
If you need some time off, even an hour or a partial day, you are able to schedule it in advance, many times this is not discussed in orientation but if you schedule your absences it does not count against you.
Management may ask for documentation at any time so be prepared, but usualy not until an absence of more than 3 days.