Suppose that a super advanced alien civilization comes to Earth. We don't know why they came and they don't really provide much information. Their species comes to Earth only for a few hours and then is off on the way to its homeworld. We didn't get to know anything about them, except that: We learned that they came from a far away galaxy group just like our own local group of galaxies, however they didn't say how they managed it. So now we know that there are possible ways to travel to other galaxy groups, but what could they be? How would scientists try to explain this?
We could deduce a lot by observing their arrival and departure.
Suppose I am an Amerind in 1492. Suddenly these bizarre people show up, with all this advanced tech. Where did these guys come from? Definitely we will keep an eye on them while they are here and pay close attention. We will watch when they leave. We will note that some of these people wear a material we do not know (steel) but that their giant boats look like they are made of wood; we know that. We will note that the small boats are propelled by men with paddles - we are familiar with those although these paddles are used two at a time and seem attached to the side of the boat - we could copy that now and give it a try. The large boats have no paddles but have big things made of material that fill with air and seem to push the boats - something new. We could copy that too. It is not clear how these big boats are steering.
So too the alien visitors. The nature of their arrival and departure will give clues. Did they suddenly appear in our space? What sort of radiations accompanied their appearances? Did a big ship stay in orbit and small landers come down, or did the big ship itself come down? Or did it just appear on the surface? What kind of interaction was there between the atmosphere and the stuff of the ship? What happened to the soil under the landers? How did they look to radar? Were they warm? Did the ships give off some sort of material or radiation involved with propulsion?
Just as the recordings made by Cassini or some other probe are studied minutely over ensuing years, recordings would be made of every aspect of the aliens appearance, visit, and departure and these would be carefully pored over by thousands of people, looking to deduce information about the alien tech.