Three times this year I was randomly selected for extra screening after I had gone through the xray machine. (I have pre-check and the buzzers and bells didn't go off until I was a few feet passed the machine. I even got a nod from the TSA agent as if everything was okay. And then the buzzers went off) Once they only wanted to check my shoes for explosives, once they wanted to swab my bag for explosives, and the last time was to have me go through the more intensive lookie loo machine.
Each time they said it was just random. But three times in a row? And at two different airports.
By the way....most UK airports, and Heathrow in particular, are the worst. Extra thorough and extra slow. One time at Heathrow my carry on was pulled out for extra screening. The screener kept walking away to go argue with her coworkers as to who should get the next break. Fifteen minutes had gone by and she was still not finished with me. Luckily a supervisor walked by, saw the open bag in front of me and asked if I was being helped. I told her what was happening and she quickly got my agent back to finish.
In Guernsey, I had a half ounce bottle of hand sanitizer that caused the security agent to freak out. . The label was in Norwegian--I bought it in Oslo--and she kept saying "I don't know if this is allowed." I told her its hand sanitizer. She insisted on taking into the back room to have it checked. A few minutes later she walked out with her supervisor who handed it back to me and said it was fine. "I know," I said. "It's hand sanitizer." (Pre-covid)
At Gatwick, I had two agents freaking out and not sure what to do about a travel sized roll of duct tape. They had never seen it before nor heard of it. They wanted to know what is is used for. I told them. They weren't sure it's allowed. I told them it was. They said okay, they would allow it this time.
Once at London City, my carry one was pulled aside and even the agent searching my bag had no idea why I was stopped. He said sometimes if there are too many electronics together they can't make out what each is.
In Kirkwall, Scotland, there is one security line. On the day I was trying to get through, they were training someone. My flight was scheduled to go in a few minutes and most of the passengers still hadn't been screened. (It's a small airport.) Someone from the airline came over, said something to the security agents, and the trainee was removed so a more experienced agent could get people through the line.
Italy seems to be the most relaxed. Once in Milan, I was on line behind a "hen" party where the bride to be was blindfolded so she didn't know where she was going. The security agents on that line, all female, were loving it and preoccupied with this. I put my bag on the belt and watched in go right through the x ray machine without stopping or anyone looking at it.
In Venice, it seemed all bags were being screened for maybe a second or two while going through the x ray machine.
Berlin was efficient. Once my carry on was pulled aside. The agent, who spoke English, pointed to the xray of the insides of my bag and asked me about an item. It was a stick deodorant. I told him and he said okay. Never had to open the bag.