How did the man spend his last day in London?

I had spent my last day in London visiting friends, taking pictures and doing some last-minute shopping. Among other things, I had bought some presents: a shirt for my brother, a woolen blanket for my sister, and a battery-powered alarm clock for my father. After traveling in a crowed bus and waiting in the noisy airport building, I was glad to be sitting in the plane at last. In a few minutes now we would be asked to fasten our seat belts and to stop smoking, and then we would soon be up in the sky on our flight to Berlin; But I had been mistaken. Ten minutes later, instead of enjoying the beauty of evening sky from high above the clouds, I was sitting in a smoke-filled room with an airline official and police officer at my side. They asked me to open the suitcase and spread out its contents on the table. I did. The moment I place the alarm clock on the table, two officials looked at each other and smiled. Hearing the clock ticking away, I suddenly understood someone must have heard the ticking noise from my suitcase and thought there was a time bomb hidden in it.