Two of my children had covid over xmas. My eldest dd discovered it through the lateral flow test she has to do for work. She had no symptoms at that point. When she did get symptoms she was a bit wiped out, had a headache and sore throat but said it wasn't as bad as a cold. After five days she lost her sense of smell and taste. If she'd waited until this symptom to test she'd have passed it on to work colleagues during the week and family member on xmas day.
My younger dd had a really nasty headache about a week after her sister had her positive test and a sore throat. She developed cold like symptoms and lost smell and taste about three days later.
I didn't test my youngest dd but the only sign she possibly had it too was red toes, which fit the photos I've seen of covid toes.
I tested negative when my daughter did the main test after the lateral flow one. I had a cough which may or may not have been covid related, but this was more of a typical symptom than any of the children had. I tested again at the end of the isolation period to see if I had picked it up or if I was safe for work again. I tested negative again.
I don't think the symptoms people are looking out for are the first symptoms and that is why it's spreading so much. Get a bad headache and shrug it off as a hangover, or hormonal, or migraine and you've missed a sign it could be covid. Lose your taste and smell and you could have passed it on for several days prior this before getting the test.
Fortunately for my household it was mild but if we'd seen vulnerable relatives in this time frame it could have been a very different story. Not sure how I didn't catch it though.