Plumbers - nothing broke - just a question

I live on top floor in a condo complex near where the hurricane made landfall this morning. The wind was 40-60 mph for a few hours w/higher gusts. During that period, the water in the toilet bowls would gradually empty - and I have noticed that happening during previous tropical storms. In fact, the bowl water is not calm then but is "moving".
Is it vent/stack related? Something else?

Thanks in advance.