@bshal
Not bad. The owner could have gone out yesterday and came back to get something? Doesn't mean it was unoccupied for a while. Actually, I will give you a point.

I guess this question will have too many possibilities, so let me do the reveal....

..drum roll.....

The detective turned the tap on. For few seconds he could hear gas coming out, making whistling noise. It was then followed by stream of water coming out all at once. The bathroom was thus not used in a while and he deduced that the house was not occupied. (Now don't tell me the owner preferred taking a leak outside his house. ;).