Comes as no surprise.
When working in security I once met an MD who boasted that no-one would ever guess the number sequence for his offices as he had set it himself. I waited two days and got my Sec to ring his Sec and tell her that we wanted to send cards to our customers on their birthdays and could she please tell us his birthday and age. She did and the next day I was in before he knew it.
If you want to try a spot of breaking and entering the most common combos are 12345 or 54321, the diagonals from left to right or right to left or the Security Manager's DoB.