Upstairs floor drain***

My two story house built in 2013.
Two floor drains.
A. In laundry room.
B. In HVAC/water heater closet. Ac does drain into this one.

Both have standing water in them about 3' from top.
They have not over flowed and when I dump water in them they will not over flow but should they be full of water?
1. I have ran a 25' snake through both drains ( did not help)
2. I have plunged both drains the water level does drop but comes back overnight. I can hear the air and water in the opposite drain when plunging.

Starting to think I have a vent problem.
All other drains in house work fine.
Any advice would help thanks.

The floor drains have traps. Without the water in the traps sewer gas would enter the home.

100% normal. No cause for concern. Its a p-trap, just like the little ones under all your sinks.

Well.... That's great news. thanks for your input. I feel like an idiot I have been messing with this for days. Guess you can't fix something that's not broke.