It sounds like he might have taken the 6 to the F from GCT. LE originally stated they were following leads around
Cherry and Rutgers Streets then
Jay and Sands Streets.
I don't know if those were determined from sightings or mobile geolocation. Cherry & Rutgers coincides with the F train's path, so if it was mobile geolocation, he might have just been on the train at that point. If they have visuals, perhaps he got out at the E. Broadway stop only to find that it's quite difficult to get up to the bridge's pedestrian walkway from there.
York St. seems the likeliest exit point, which is right by the stairs to the bridge's walkway. It's about a 10-13 minute walk to the center of the bridge. If Thomas's last known location was on the bridge at 19:06, then it must have been a location fairly well along the bridge if the substantial splash was recorded four minutes later. Oh, Thomas. Society failed you.