Today the President signed an extension on the Patriot Act using a device called autopen (the merits of the Patriot act are not of concern here, only the circumstances of its signing). The reason for this is, the President was in Europe at the time the bill was approved by Congress and ready for a the Presidents signature. According to Article I Section 7 regarding the passage of bills into Law it states:
Every Bill which shall have passed the House of Representatives and the Senate, shall, before it become a Law, be presented to the President of the United States: If he approve he shall sign it, but if not he shall return it, with his Objections to that House in which it shall have originated, who shall enter the Objections at large on their Journal, and proceed to reconsider it. If after such Reconsideration two thirds of that House shall agree to pass the Bill, it shall be sent, together with the Objections, to the other House, by which it shall likewise be reconsidered, and if approved by two thirds of that House, it shall become a Law.
Article I Section 7 also states:
If any Bill shall not be returned by the President within ten Days (Sundays excepted) after it shall have been presented to him, the Same shall be a Law, in like Manner as if he had signed it, unless the Congress by their Adjournment prevent its Return, in which Case it shall not be a Law.