NBO – HRE KQ flight as a passenger about a decade ago. Felt a soft but clear push in my back at rotation so I explained my neighbor (US Marine) that we were going back. After circling for about 20 minutes around town and being catered for a couple of more hours at the ground we boarded the same 737-800 again. Of course some passengers were a little stressed and exited. During taxiing out I suddenly heard a sharp hissing sound and was starting to accept that I might have to stay overnight. It appeared that a couple of rows away from us two smartly dressed middle-aged gentlemen were standing, both with yellow life-jackets on, one inflated. The purser came to them and calmly explained that it was not necessary to rehearse the in-flight safety demonstration.