This just happened to me as well! Two crewships infact!
In my case, the mission completed normally, and the railjack parked normally, but I noticed some creship markers on my hud showing a few room away in the dojo...
I was able to approach them and even board one, there were no grineer inside, and they appeared mostly docile, though they did move around outside the walls of the dojo.
Here's an imgur album with annotated screenshots describing the details: https://imgur.com/a/3xaVWyD
Ironically, this happened right after a match that required the host to leave (triggering a host migration) because crewships stopped spawning causing the objective to not be completable. (but that's a whole other can of worms).