I don't know who design the lab but it doesn't make any sense from a design stand point. You've got to understand that it isn't a single room but an entire building so there would be ventilation ducts, building supports and other rooms in the basement.
Another quick paint over. The player has slept and discovered that the alien has mutated to spit acid and has melted the glass and then escaped into the ventilation, The player would then climb up using a ladder or chair to see it just disappear at the top of the shaft.