signed at, signed in

Both signed at and signed in are correct. However, in legal documents such as contracts, conventions and agreements, the traditional expression signed at is more common.

• The Treaty of Paris was signed in Paris on February 10, 1763.
• International Convention Against the Taking of Hostages. Signed at New York on 18 December 1979.

The prepositions in and at both indicate place or location. However, in emphasizes the idea of being inside or within boundaries, while at expresses exact position, such as a point on a map.