I own a tri-level and in the upstairs bathroom(and it always seems to be late in the evening)--when the toilet is flushed or the sink is run for awhile ---you can hear what sounds like anywhere from 5-10 drips in the ceiling--but there are no water marks. Am I imgining things because the only thing I thought was up there was the vent. Any help would be greatly appreciated---thank you

If you see no reason for water damage anywhere (and if it was a leak you would notice it in the ceiling !), what you are probably hearing is water dripping within the drain.