Thanks all.
We went to look at the house again today, and noticed ceiling in one room on the second floor has water damage sign. So I guess the tiny holes were drilled to release the moisture. Not sure when did the leak happen and doesn’t look like will get an answer from the seller...
The second floor bathroom had a leak once upon a time too, which might be why the smoke alarm on main floor also has those holes drilled to release the moisture.
Then, also notice basement ceiling behind the stairs also has some water marks. This is in the middle of the house, so it is really bizarre to see the water stains...
The stained spots are all dry, so the leaks must have happened some time ago. But with all these water stain markings, I am hesitant to make a decision.