Alien space drones. Doesn't have to be FTL or even close. Just takes time.
sigh .. no.
i will explain why.
there is an abstract concept of maximum expansion due to, to make it simple, "reality".
imagine you need to go to a single point in space, very far away; you build a colony ship and send it there. it will take an enormous amount of time, generations, but eventually it will get there.
the ship travels to its destination, but after a non-descript amount of years, another ship pulls up to its side: it's a ship you also built, years later. but due to the increase in technology, it travels faster.
after another few year yet another ship arrives, it also left later but caught up due to the faster speed.
this keeps repeating, until technological development at the source halts.
in practice, there's distances, that are so far, that you get there sooner by *not* going, rather than by going.
it's not viable for anything that has mass and must also travel through spacetime to go anywhere outside of a single star system.
now, if the intelligence that decided this exploration was going to happen, is not organic, then sure. artificial lifeforms might have an unlimited lifespan, so 5000 years of space travel could be totally reasonable. but to animals, it's not. even if the drone is autonomous, there's no point to send a drone if the culture that sent it is unrecognizable by the time it arrived.
Again, we would need to assume that this is a lifeform that has reached a point of development where no more discoveries seem possible.
This leads me to speculate that higher-technology cultures develop internally, rather than externally. Rather than going to the asteroid belt to mine precious stuff, we would just generate it here.