The Next Generation episode "Schisms" begins with Riker being tired. Over the course of the episode, mysterious signs start to amass, with crew members missing time and showing traces of surgery. It is discovered that aliens
are abducting crew members to experiment on them and delete their memory of the events.
Geordi realizes the cause:
the signals from the modifications I made to the sensor array, some of them dig pretty deep into subspace. Maybe they caught somebody's attention.
So, sensor modifications attracted the aliens in the first place. The sensor modifications that Geordi is talking about were introduced in the beginning of the episode:
I think we've found a way to speed that up a bit. We've been testing a way of channelling warp energy directly to the main deflector grid. It should enhance the long range sensors.
So, at this point in the episode, the sensor modification has not yet been conducted.
But, wait, Riker was unusually tired even before that. How does this go together?
In theory, it's possible that "we've been testing" was already enough to attract the aliens. Still, the rest of the dialogue definitely makes it sound as if the actual sensor modification was not yet in place: Geordi's initial estimation of how long the mapping will take —
It's going to take three days just to map out one tenth of it.
— implies he is not yet certain about the effectiveness of the modification (which he probably would have tested somehow, had there been a real test of any kind already). Furthermore, Data and Geordi state:
The modification would increase our sensor efficiency and imaging resolution by twenty five percent or more.
We'd channel it through the EPS mains on deck four, near Cargo bay four.
Both say would, as if it's something that is yet to happen.
(Quotations from the transcript.)