If you leave something at airport security you should contact the airline at their customer service number or visit the customer service desk at the airport.
You can also contact the TSA lost and found and file a claim for the lost item on the TSA website as soon as possible.
Provide as much details as possible like the date and time you lost the item and a detailed description of the item, which can help them match the item with lost property in their system.
If you left an item at airport security or on the airplane immediately inform airport security or staff so they can look for the item.
If you've left the airport and they found the item it can be shipped back to you at your expense or if you're near the airport you can come back to the airport to retrieve the item.
The reason why you can't get back on a plane after getting off is mainly due to security and safety.
Getting back on a plane after getting off without going through security again compromises the security and safety of the other passengers and airline employees and flight.
If airlines allowed passengers to go back onto the plane after they got off without going through security again, it would compromise the safety and security of the flight and airport as well as efficiency.
That would mean that even though a passenger had previously passed security checks and has flown on the plane without any issues, the passenger could go back onto the plane with something dangerous such as a bomb, gun knife etc or could decide to in that time plan to hijack the plane eventually.
So airlines have to stop people from going back onto the plane to get items after they've gotten off.
After you leave the secure aircraft or plane area, you are now then outside of the monitored and secured zone and any re-entry into the secured area and plane would require that you go through security again.
Also aside from safety issues and security issues, allowing airline passengers to re-board the airplane even to look for something or retrieve something would also disrupt the airlines tightly scheduled flow of aircraft turns and the necessary procedures, like the accurate reporting of passenger and baggage counts, which is also crucial for the departure of the flight.
Aircraft turnaround is also a quick process and having any disruption can cause airline delays and allowing someone back onto the plane also disrupts the flow of other passengers, baggage and crew and makes it difficult for the airline to stay on schedule.
If you've left something on the plane you should let the airline know about it immediately.
If you're still at the airport, return to the gate or speak to an airline representative at the gate and inform the airline staff about the item you left on the plane and the location of the plane.
A staff member can then check the plane for the item or items before the airplane departs and if they find it they can give it back to you.
If you're left the airport you should contact the airport through their number or through their website or app as soon as possible and fill out a detailed lost item report.
Include the flight number, seat number, date, time and a thorough description of the item, which includes the bran, color and size.