air can be a big enemy, and with that comes odor transfers and freezer burns.
this is the ultimate airtight test. fill a pot with 2 inches of warm water, about 120 0F. cover with plastic wrap. wait till the pot reaches room temperature, about 75oF. the decrease of temperature will produce a vacuum and the plastic wrap SHOULD start to tighten.
furthermore, place the pot in the refrigerator, overnight, and the plastic wrap SHOULD form a concave lid. if this isn’t airtight, then what is?