So, if I'm right, Linhars, "si det" would be "who knows" in context of something that is not widely known or not known and does not require a strict answer. Usually interchangeable with a lot, but not always. E.g. How many grains of sand are there on the beach. - Who knows! Or, "who knows when this isolation's gonna end".
But hvem vet would be when a teacher asks in the clasroom: "Who knows what's the capital of Norway?"