Rope Entrances - Squirrels can easily get into attics by climbing up a rope that is escape-proofed to the roof.
Window Nests - If there are openings in your attic window, squirrels will use them as access points to your attic. Once inside the attic, they can easily maneuver around and through the spaces to reach the stored items.
Pipe Ends - A squirrel may find its way into your attic by scaling a metal security grill protecting an air conditioning or heating pipe near the edge of the roof.