I think there are countries where there is evidence for undetected cases- for example in Spain where they found a case after someone had died, and that person would have been infected before the first known case on the Spanish mainland. Obviously that person had caught it from somewhere and is likely to have spread it to someone.
In some countries, like the USA and the Netherlands, they were finding cases of community spread very early on in the testing process- now they are playing catch up and trying to catch as many cases as possible, but it is likely there are a lot of undetected cases in the USA in particular.
There are also countries like Thailand with a sharp spike in cases of "viral pneumonia".
In the UK we've got none of those things. Certainly there are a small number of undetected cases leading to community transmission, as well as people who've come back from Europe and aren't showing symptoms yet. It's impossible to prove, but I think the number of undetected cases is less than 100, not thousands as PPs have suggested.
It's almost reassuring to tell yourself that loads of cases are being missed, so the virus is not as bad as it seems, but in the UK at least, the evidence really doesn't back that up (unless you assume the government are lying about everything, which I don't).