What has caused my basement to suddenly start smelling like a litter box?

I do not have a cat and lived here for a number of years and never had this smell occur before. Suddenly my unfinished basement reeks of a smell that I can best describe as cat urine, or more specifically a litter box. Their is not outside access for a cat to have gotten in there and I did not see any evidence of any mouse, or other rodent, poop. We did have a decent amount of snow on the ground that start to melt this week as the temperature has finally warmed up a bit. We live in Southern NJ.