Tire pressure decrease when the bike is idle I ride a Btwin MyBike 7s.

Yesterday morning I noticed (for the first time since purchase) that in my rear tire, the tire pressure has decreased to a flat. I thought it was a puncture, so I refilled the tire using a hand-pump and poured water over the tires expecting bubbles but didn't get any.

Then I took it for a ride, 20 kms and came back - nothing happened.

Then I came back in the evening to notice that the air pressure has decreased again, almost half. Then today morning it's a flat again.

Is it a puncture? I checked the tire's mouth valve, it's not leaking any air there. Should I get the tube out and check for a puncture?
Answer: Your tube has a slow leak, this cannot be found by pouring water on the outside of the tyre.

Get the tube out of the tyre, which means unmounting the wheel and maybe releasing the brakes.

Inflate the bare tube to about 5 PSI, no more than a couple of strokes of the pump.

Slowly submerge the tube, one section at a time into a bowl of clean plain water.  Look for bubbles to form and rise to the surface. 

Since the tube takes hours to go flat, it won't be a big hole  Once you find it, mark with some tape or chalk or something.

Dry the tube, and run through the patching process.  Remember to buff the tube lightly, apply enough fluid to cover an area bigger than your patch, leave the fluid 5-10 minutes to dry completely before applying the patch, and to roll the patch on hard using a round coin or other pressure - fingers aren't enough.

Then you can inflate the tube to 5 psi and check for more leaks... there can easily be multiple holes.

If your valve is leaking, just buy another tube of the same dimensions and fit it carefully.

If you've never done this kind of thing before, ask family and friends for some hands-on help.  Its not hard to do, but some things are better watched than explained.