Marks after rain
If dampness follows rain, inspect roof junctions, facade cracks, windows, and drainage. Water travels along beams and slabs, so the indoor mark may not match the entry point.
Marks after water use
Dampness after showering or sink drainage may involve joints, drains, or waterproofing. Growth without rain or use may justify supply-pipe testing.
Test by area
Photograph and record timing. Controlled spray tests must isolate one area rather than wetting every junction together.
Sources
World Bank, Urban Development in Phnom Penh; World Bank/GFDRR material on Phnom Penh flood risk.