Spoiler alert: Mice do not leave on their own. If house mice stop being noticed it is most likely because they died as a result of a mice treatment from one of the neighbours. So you may want to speak with your neighbours and find out if they did some mice pest control.
However what is most likely to happen is that the mice infestation has gone down and now continues at a level that is too low to be noticed. Mice remaining mostly at void space level and in small numbers would give less opportunity to be spotted. When carrying out mice control in London, we are often told that the client had noticed mice activity for the first time ever. But as we open the kkckboards below the kitchen units, we find at times 100s of droppings.
Usually mice infestations that happen suddenly can often be associated to another event taking place at the same time. It could be a neighbour refurbishing, or doing some spring cleaning. The opposite is true as well. If you would have been suffering from a mice problem to a certain level, and that they suddenly disappear, you should be able to find out that the neighbours below carried out a round of mice eradication or possibly introduced a cat, or sonic devices.
In some instances, we have difficulty to get across some tenants who openly recognise that they have mice, but dismiss it as inconsequential as they believe that mice always come and go. Hence they would not accept the mice extermination London cost as being justified.
The only reason why some mice would want to leave in their own, is in case of very high level of infestation. Simply they would find it easier to move toward the less infested neighbour where competition is less. But by no mean would they all leave of their own volition without any competing force. Mice behave as a population, and when you see one, you know that more will be coming.
So stop wondering if mice leave on their own. Rather focus on how to tell when all the mice are gone.