If someone has changed the locks on your vacant flats, you have a few courses of action. You can immediately change the locks back yourself with witnesses, though this may lead to disputes. Alternatively, you can file a police report (FIR) or complain to the society. Experts confirm that adverse possession cannot be claimed in this situation, as you were unaware of the trespass until your recent visit. Installing CCTV is also recommended for future security.