I live in a condo on the third floor and the building was built in 1985. Recently I noticed that the water level has decreased significantly in the toilet bowl. I checked the tank and it is filling up as it should. I have observed no leakages around the toilet. My downstairs neighbors have not complained about a leak either. What could this possiblly be attributed to?

Make sure the bowl fill supply line is in the overflow tube, that is how the bowl fills. Make sure the line isn't kinked. Make sure the fill holes in the bowl rim aren't plugged. Take a coathanger and probe each hole. Some times a plugged vent will cause the water to be sucked out if a toilet from another unit is on the same stack. Have your condo ***'n look into that.

All good points by ma0641,

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ma0641 Make sure the bowl fill supply line is in the overflow tube,

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This is the small tube that runs from the ballcock to the white overflow tube. (see images). This tube fills the bowl as the tank fills.
Do you hear any gurgles when flushing or draining? If so you have a blocked vent but I'd check the fill valve first. Good luck, Tom