I had this problem in a flat, also. I found one mouse one day and took him outside… naively. As you will have concluded from the other comments here, it wasn’t just the sole mouse. As for solutions - mouse traps and peanut butter. Definitely not humane or enjoyable so I understand if you’d rather not. It gave us an idea as to how many were around (we killed 20+ and we honestly saw one or maybe two). In the end, it was solved not just by keeping a super clean place, but by inspecting every single nook and crevice physically possible. Any doors had door blockers underneath (you can buy long strips online, I can’t remember what they’re called though!). We stuffed holes around the water pipes with metal mesh wiring. We filled any tiny holes leading outdoors (again, water pipes or waste pipes) with foam insulation. What was particularly effective was looking underneath all the kitchen units, moving all appliances, big and small. Any hole is a goal, as they say. Make sure you do it safely and don’t cause a fire hazard. But I imagine doing this process alongside bleaching, cleaning, leaving no food and a few weeks/months of traps (or humane equivalent) will get the job done. Don’t just hire a service to come and place a few traps or poison, it honestly won’t do anything for the long term, I can promise you.

You've very likely already got a multigenerational mouse population in your home and across other flats in your block. Blocking entry points. Fill any gaps or holes with wire wool.  Infestation. A rodent specialist might be best (e.g. Rentokil). Or contact the council if it's the whole flat complex.   Quick. Spring trap***s*** (plural) loaded with peanut butter - if you're mentally prepared with a strong stomach to hear the snap and then empty and dispose. Empty. Reload. Repeat. For days or possibly weeks.  Effective but slow. Poison and box. Poison is taken back to the nest and kills the young too. Be prepared for the chance they'll die under your floor and honk the place out.