1. Is there any way the supplied exported txt file could have introduced this segment?
Let's assume so. I suppose your TM worked just fine until then.
2. If not, is there any way of searching (Maintenance?) for a particular segment number in the TM?
3. Failing that, is there a way of searching (again Maintenance?) for an empty source string so I can delete the corrupt segment?
Segment numbers are, I think, not fixed. I don't see how you could search for them.
Empty souce? Maybe; it's like sending the maintenance crew in to look for "nothing, guys, just look for nothing."
Maybe there's a good way to define "nothing".
Anyway: I'd try by creation date; the date you imported at least some of the outsourcer txtfile; maybe (so many maybes here) that'll turn up the culprit. (I'd delete the entire lot, instantly.)