My sister and I were birding there yesterday, too. We were on Gage Road not much earlier than you, but saw no raptors, as I recall.  We did see a perching hawk, maybe from Jersey Street, she was uncertain of and I wonder if it was your prairie falcon. She is a good birder but didn't think it was quite right for one of the usual species. She considered a merlin, as well.  One thing it did was to make a soft 'chu-rp' call and I wonder if that is something a prairie falcon, or another raptor, would do.  

We sighted 4 snowy owls in the area.  One snow goose near a barn.  Many birds visible in the distance on Lake Champlain from the DAR Park.  

Eric