Since your probation officer was not in the office, your email was a good idea. Just a suggestion. Another suggestion would be to obtain some sort of documentation from your work that you were actually at work.
Follow up with a telephone call to you PO and for your records note the time and date of call in the event that you left a voicemail message. If this were my probation officer, I would keep trying to reach her/him to make sure that you were good with them and if they needed anything from you regarding your work that day.