Swift questions

Work and weather stopped me seeing much so far this week apart from a Speckled Wood butterfly on Tuesday and 65+ Linnet at Smith's Field this morning.
That is apart from a late Common Swift this evening. I see the odd late Swift, even later than this one but what I don't understand is what it was up to. Most late Swifts are fly-overs/ fly-throughs but this one was happy to be feeding around the train station and the Avenues area as long as I watched it. Now this area is the main brreeding area for the species in Chester-le-Street and there could be well over 75 pair breeding here mid summer. But they all left more than three weeks ago. Pesumably this could not be a late bred youngster as I pass this area most week days once or twice per day and there have been no sign of Mummy & Daddy or any Swift for, like I said, three weeks. So is this just a bird from further north that has stopped off to feed and perhaps even and roost,  over this area by coincidence or is there something much more instictive happening? I couldn't see it well enough to tell if it was an adult or ypung bird but what made it decide to stop off here, presumably on it's journey from the north? Was it a bird born and bred here previously?  Plenty of questions,bu no answer.