Warped siding letting mice in
Re-nail the bottom of the T1-11 to the sill plate every 6 inches. I use Simpson StrongTie ring shank stainless steel siding nails. Be sure you know where any water pipes and drain lines are in the exterior wall, and use the length of nail that would not go much more than 1 inch into the sill plate. You could go 1.5 inches into studs low on the walls where there would be no pipes or wires.

Simpson StrongTie stainless steel siding nails