A leak of a litre or more a day into the fabric of a building should be quite noticeable if it continues for a few days.
Based on your observations, I suspect that the PRV is most likely to be responsible. You could confirm this by connecting a pressure gauge to the DHW and see how it correlates with your 'leak'. The mains inlet pressure should also be checked. PRVs need to have a reasonable drop across them to work correctly. If the incoming pressure is not high enough, e.g. because all your neighbours are having their baths and showers, then the cylinder will be left at too low a pressure and will top up once the inlet pressure rises again.
You do sometimes get leaks that are temperature sensitive, e.g. open after hot water has been run due to the increased temperature and close as the pipework cools, but they are not common. And, as I said above, a litre a day from your DHW is probably going to be fairly obvious once you start looking.