Water Leak on Wall

Since August, damp patches have developed on two walls next to my bathroom and kitchen. On one wall, the patch appeared halfway up and spread downwards; the other wall became damp from the bottom. I live on the second floor of an apartment building, with neighbours above and below. A leak detection company found that water could enter behind a damaged, loose tile and the adjacent grout in the shower corner. The report can be seen in the attachment. I tried to seal the area with Bison water-resistant silicone, but the problem has continued. Photos from before and after the repair attempt are attached. A construction dehumidifier ran continuously for a week. Most of the walls became visibly dry, but some areas remained damp. After I switched it off for one day, the damp patches grew again. My suspicion is that water enters through the bathroom tiles or leaks from a drain when water is used, rather than from a pressurised supply pipe. The water meter showed no movement during three overnight checks. The leak detection company also found no indication of a supply-pipe leak in its tests and reported no fault in the shower drain. I cannot see any other obvious damage in the bathroom, so the cause still needs to be checked. I am looking for a professional who can identify the source and carry out a lasting repair. I can share the full leak detection report with the company I select. Please describe your relevant experience and indicate your estimated cost, including any inspection fee and the likely repair cost if possible.