The wife was in the shower (no rule 1) and both the downstairs toilets started gurgling. They are all 3 close in proximity and share a vent. As the toilets gurgled they lost water, indicating the water was replacing the air in the drain. Both toilets flushed fine and the shower was draining normally. The sewer line going to the septic had flowing water, so I suspected the air vent was plugged, but I can't repeat the problem now. I would think if the vent was plugged it would happen every time? Is it possible maybe an owl or something was sitting on the vent? Other possibilities?

You have a partial clog somewhere in one of the drain lines. It may have broken free. If you have a snake send something down and then try some Green Gobbler drain cleaner.

Green Gobbler Drain Cleaner

I don't see it often but it is possible that one of your plumbing vents is plugged on the roof, birds, wasps, whatever….have you checked that out yet?