The reason why TSA would swab your stuff is to test for any trace amounts of possible substances that could be used to create explosives and to test for any trace amounts of explosives themselves.
Some of the possible explosives that can be detected through swabbing of your stuff is glycerin, nitrates and other chemicals and is a standard security measure to prevent terrorism by detecting any explosive residue on items which may have come into contact with such materials, even if it was unknowingly.
The swab after swabbing your items for possible explosive residue is then placed into a machine to identify any chemical composition and if a match is found, then you will often be subject to additional screening.
Airport security may also take your phone so they can check for any trace amounts of explosive residue with a swab and even to verify that the phone is a functional device instead of a bomb like object or in cases in which you refuse to unlock the phone for a swab or power on test.
TSA is not permitted to search your phone's content without a warrant, but U.S. Customs and Border Protection agents at borders can search your phone or devices without a warrant or suspicion.
Airlines also don't allow you to get back on a plane after getting off mainly due to security and safety reasons unless you go through security again.
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.