I've been having this issue for a while -- nothing in my review queues, but the red notification dot is there:
Over on meta.stackexchange.com I found a post discussing the issue: Experiment: Review-needed indicator logic for sites that sometimes have empty queues
@JonEricson recommended I post here and ping them there to bring attention to the problem.
Wondering if anyone else is having this issue, and what can be done about it?