"What I don't understand is how the virus can circulate unnoticed for so long, when we've seen 2 big peaks involving large numbers hospitalised/dying?"
It's simply the nature of exponential growth.
Don't think about this too much, just give an instinctive answer... would you rather have a penny a day for a month, doubled, ie day 1 = £0.01, day 2 = £0.02, day 3 = £0.04 etc. Or a million pounds straight up?
Did you choose the million pounds? Here's what you could have won on day 31
£10,737,418.24
If I halt the doubling a smidge early on day 27, you'd get £671,088.64, a big difference to the nearly £11 M. Most of the days of the month the numbers are mundane, unimpressive sums of money. Same with the numbers of viral infections in a population, numbers are very small to start with in a country with a population of c 60 million. Case 0 lands at the airport shedding the virus, infects the passport control officer and the taxi driver. They each pass it on to two other people and two more each in turn. It's september, they likely have reasonable vitamin D levels as it's late summer, so most of them just notice feeling a bit off/fatigued for a few days and a scratchy throat. Continues to spread into October, a few tens of people now. Levels of vitamin D are declining, people's potential to get more sick is enhanced. Cases start showing up in GP surgeries with bad coughs/pneumonia. The doctor thinks the couple of cases they've seen seem slightly unusual this year, but hey it's that time of year for chest infections and people can present idiosyncratically. Cases start showing up in hospital, where the radiologists start noticing unusual patterns in the chest xrays, enough to make them think hmm and casually mention it to their shift colleague, but no where near the level the threshold that the lightbulb goes on that it's a novel pathogen they're dealing with. A handful of people die, but it's mixed in with the statistical "noise" from influenza and conventional pneumonias in the winter flu season. A few weeks pass and the virus is stealthily trundling round the population, doubling every few days, there's more chatter on medical forums there's something going on... meanwhile reports are coming out of China of a new virus. Then the "oh shit" moment hits in Europe where the tipping point in the hockey stick graph is reached and turns to a steep upward trajectory, hospitals suddenly become overwhelmed with patients.
There was an outbreak of an mystery cough/pneumonia respiratory illness that killed two residents in a nursing home in Virginia in JULY 2019. 53/263 became sick. Normally they only see such respiratory illnesses and deaths in winter. Could quite possibly have been covid, and small pockets of it were very slowing spreading, gradually picking up momentum.
edition.cnn.com/2019/07/11/us/virginia-retirement-community-respiratory-illness-outbreak/index.html
wjla.com/news/local/mystery-virus-greenspring-retirement-cdc-va