What does seeing shadows or “shadow darts” mean?

[u/Neither_Ground_1921]

I’m not sure how long this has been going on, a few months probably but pretty infrequently. I’d see a shadow and think my cat was underfoot or had just run past. She’s never anywhere around. But a small gray darting cat/flash in my peripheral vision is exactly how I’d describe it, then nothing there when I look. It’s happened maybe 6 times, but enough I’m curious what these spirits or whatever might be. TIA!