I'm with Mc H on this one, if you tapping into a sewer line it should have some type of gully pot in there.
I'm sure you have concreted over the joint well enough as you know what your doing but I see jobs like that daily where rats dig or chew out as they have just dropped a new line or pipe into a foul line. So many builders messing with drains haven't a clue, they just cut a square hole and run a pipe into it, then cover with earth. Even covering with gravel or sand and cement often fails after 10 years of rats scratching at the joint. Obviously I'm not accusing you of doing half a jo