Humans entering space and realizing we are so small. We are mice compared to these giant races with their advanced machinery and technologies and experiences beyond us- except that we're humans. And our engineers dive into the new tech and once we learn the principles we also soon realize how Inefficient everything is. Their "microchips" are the size of cars, their storage drives are basically buildings, and they somehow store less data than ours. So, human companies take advantage, and tech starts rolling out. Massive and there's a lot of wasted space so that it can be managed with larger hands/pincers/claws/tentacles, but also so much more efficient than anything the galaxy has seen before.
Human technicians start hopping ships and upkeeping the general maintenance, the stuff that most aliens put off or don't notice because they never access the crevices of their ships. As human companies become more popular and lead the tech world in everything from warp cores to game stations ("it's so compact! How are the graphics so good?" Says a 60' tall grimbleback, holding a new VR headset that has all of its components included because it's so BIG by our tech standards), soon many things have accessibility ports for humans to be able to use as well. This means that these shiprats hoping ship to ship cause such a huge improvement in everything running smoothly, and there's a huge downtick in pests on ships because those "pests" are not only big enough and aggressive enough to bite a pitbull or a person in half, they're invasive to so many planets and humans hate nothing more than dog killing planet overrunning monsters.
All the while, from the Aliens perspective, humans are an elusive race that don't fraternize much with them. You almost never see a human as most places aren't exactly safe for the little things to run around in. They do export so much stuff though, and the custodial staff at the Central Galactic Outpost insists that there's more humans around than any other race if you just know where to look.
And sure it's somewhat known that some of the little daredevils hop ships and help out in exchange for room and board, usually without permission, but that can't be that common, can it?
Maybe your ship is running better this cycle ever since you stopped at the last station, that just means that tuneup was better than you thought. And maybe for some reason that program you were working on last night is finished when you wake up, but you're so tired maybe you finished it before you passed out. Somehow that faulty light in the galley has fixed itself as well, which is odd, but maybe the Engineer finally got to it. You'd know if there was someone else on your ship.
Right?
... You leave a little bowl of berries out as a thank you, just in case. You're not sure what humans like but you've heard they have a sweet tooth.