^ they should be able to detect that time travel is taking place, surely they can scan for the properties of time travel and see when it is occuring
But it is a part of History, it's not something that's just suddenly going to pop up on sensors because it's already happened 500 hundred years ago! and it is an event that has already shaped their time line. They're not going to see any time line alteration because as far as they're concerned it is already a part of what's happened, to go back and alter it would be to screw with their own time line.
Detecting new time jumps and alterations by people from the future is different from seeing events that occured 500 hundred years in the past. Nero isn't from the 29th century, he's from the 24th and to the crew of the Relativity it's nothing more than something for the History books.
Imagine it's 2650, for the first time ever you switch on the first ever temporal sensor, it shows you that Nero in 2390 went back in time to 2230 or whenever. What do you do? obviously you do nothing, because to do anything would mess up your own time line and you will be unaware of what the consequences might be.