About the only thing I can think of is that perhaps there's an access above in the house. Maybe under the carpet. This kind of seems like the kind of problem you'd want to just 'work around' because I can imagine it being very expensive. Best guess is that the factory didn't connect the vent hose correctly and it popped off. When I fill my tank at a park with high pressure, it can really shoot out the overflow when it gets near full and then keep burping out more water for a while after turning off the water. (Another reason I don't often fill it).