It wouldn't be the first house I've seen where the plumbers stubbed the vent into the attic, and no one ever installed the vent terminal through the roof (like ED suggested). Or where some homeowner saw or imagined leakage around the vent flashing and had the great idea of cutting the vent off in the attic and "fixing" the roof.
Either way...I second the recommendation of having a plumber check it out. If the system is otherwise properly vented but the vent simply doesn't extend through the roof, it will be simple and inexpensive to correct. Good luck.