6 triangles, translucent, very high in the sky

My husband heard a sound like aircraft. he looked up and saw what looked like gliders but so high up they couldn't possibly be gliders. It took me a while before I could see them too. they were small triangular translucent objects close together and very high in the sky. We told our daughter but she couldn't see them. There were 6 triangles in total. My husband is very sceptical about these matters and an avid aircraft fan. He has never seen anything like this and knows all aircraft very well. They moved slowly and smoothly. After a few minutes they disappeared.