Air flow from under slab around toilet flange

I'm finishing a basement bath and have just installed the toilet flange (ABS). I broke out about a 1/2" of concrete around the 3" drain pipe so I could install the flange over the outside. After installing the flange on top of the floor, there is still about a 1/4" gap around the hub underneath (you can see it a bit through slots for the toilet bolts). However, today I noticed that there is a small air flow coming up through this gap and its got me wondering where it could be coming from? It's NOT coming from inside the drain pipe. Is this something I should worry about or is there a simple explanation?