could air in the line cause drop in water pressure?
No, if the pressure is 80 when you turn it off then it would stay at 80 even with air in the line.

You have a leak. It could be as simple as a very slow drip at a garden hose faucet or as complicated as a main line leak.

By the way, this is a very common occurrence; faucets/valves generally leak a bit as they get old. If your piping and fittings are sound then there is no acute danger. If you are worried you should carry out an inspection of your piping system and fittings, looking for tell-tale signs like damp spots, water marks, stains, etc. Also, plumbers have leak detection equipment and procedures that may help.