I already checked any leaks around taps but I do not think the leak come from them. One thing I should mention is that I worry about dripping noise from the wall in bathroom. It starts after I use hot water in bathroom. It sounds like copper expanding noise and also dripping noise as well. I asked one plumber a couple of times before but he said it is from sewer pipes and/or expansion pipes etc and it is not leaking, then he never checked the place at all.
I have had suspicious of leaks on the hot water pipe, but I found that the wet mark recently appeared when cold tap is also used.