Found on this Lennox unit that mice had moved in the A/C for the winter. Any units that have insulation in them, especially if there is insulation around the compressor, are good for mice to move in. Check your unit in the spring before turning it on to make sure they didn’t do any damage to it.
Mice love heating and cooling systems, so getting routine maintenance in the spring is helpful to ensure your system hadn’t been damaged throughout the winter.