bad smell after pumping septic tank

after having my septic tank pumped clean, i told the plumbers to replace the lids, but to leave the tank uncovered..

i wanted to make sure everything was in order...

i noticed a horrible sewage smell coming from my "floor drain" in my basement.. i could also feel cold air coming from it.. when i covered the tank up with dirt, the smell went away...

why is this?

The floor drain trap is dry, or there is no trap.

The covers were not 'sealed' so air was circulating through the tank and into the house. The drain was also dry otherwise the air would have gone up the pipe and out of the roof vent.