Pending Prometheus (Mimir) alert has no threshold set

My understanding is that Prometheus alerts configured with no “for” clause will bypass the pending state altogether and go straight to firing. Is that correct?

I ask because I’m looking at the ALERTS metric for a particular short-lived alert that went to pending although no threshold is configured via the “for” clause. Maybe there’s a small but non-zero default value for the threshold?

If it makes a difference we are using Mimir rather than Prometheus.

This turned out to be a mistake on my part: I simply mistook one similarly named alert for another.