If it's not immediately apparent I would submerge the inner tube, after its been removed from the wheel of course, in a shallow tub of water. If there is a puncture, you will see bubbles.
Besides the spoke/rim tape culprit Alf mentioned, a piece of glass may have worked its way through the tire, the rim itself may have a bur on it, or if lucky the gremlin may have simply punctured the tube and went on it's merry way. Along with a visual inspection, the two latter causes can be detected by gently running a finger along the inside of the rim and tire, Once you have repaired or replaced the tube, try and remove any future potential causes for flatting.
When re-installing the tube, make sure it is completely inside the tire by using the thumbs and forefingers of both hands and going along the length of the rim (on both sides) with the tube only partially inflated visually making sure no bit of inner tube is peeking out under the tire directly against the rim.
