Big hugs OP, its the worst feeling in the world.
We lost our 3.5 y/o daughter at the airport in Hawaii. I was putting our 1 y/o into her stroller and hubby went to wait for our suitcases and DD1 went with him. Well he forgot she followed him and when he had our suitcases on the trolley he turned around and walked back to me without her!!!! OMG. My heart literally skipped a beat when I said "where's DD1" and he yells "what? She's not with me". The airport was packed. I just ran through the crowd back towards where they had been and luckily through the crowd I could see a little flash of her pink skirt. She had run for the exits and the Immigration officials were trying to get her but she was screaming "no no, Mummy. Where's Mummy?". They had basically cornered her so she couldn't run out. It was so scary. She was hysterical and so was I nearly.