h2s sensor sen0467 issue

i built a housing for this sensor with an inlet and outlet hole and the sensor plugs in thru an O ring.  I would have thought that putting air into the enclosure the air would only exit my outlet port.  But it seems the sensor has two holes in the back of the sensor that appear to allow gasses to escape.  What is the purpose of the holes?  can they be plugged?  i can't have these gasses filling up my project enclosure, i need the only escape for the air to exit out the outlet port.  

at the bottom left, you can see to small holes.  

Cheers for any advise

jim