They would most likely continue to serve their now dead/missing masters as they had previously.
As you mentioned, Intellect Devourer's are created to " lure others into its illithid creators' hunting grounds". I couldn't find anything concrete that mentioned intellect devourer's having any kind of independence/capability of seeking out another master to server, or another colony to join. However, that could be possible.
Since that they are incapable of individual thought, it makes sense that they would continue to serve their illithid masters as that is all they know. Even if their masters were dead, they may continue to lure adventurers into the lair of their, now dead, masters as it has always done.
It is also possible that it may realize that it's master is dead/gone, and revert to purely primal instincts and just try to survive. It no longer lures people to a lair, instead it seeks out intelligent creatures and consumes the "creature's mind and memories, subsequently entering the body and using it as a new host" as a means of survival. 
It's entirely up to you/the DM on what makes most sense story wise.
At the end of the day, what really matters is what make sense story wise, or what would be a cooler/more interesting encounter. If you wanted the party to be lured by an intellect devourer to an abandoned illithid lair/city that is now filled with long dead/rotting corpses, then go with that. Otherwise, if you think they are capable of individual thought, then they could try and seek out new masters to serve.
I found this interesting point that described what an intellect devourer's purpose would be quite nicely:
Intellect devourers are aberrations, created to serve the interests of mind flayers. They’re not independent creatures. You’re not going to randomly run into one in the woods. Rather, any intellect devourer your player characters encounter will be on some kind of mission. That will affect who it uses its powers on and when.
That quote is what informed this answer, but ultimately, it is up to you and what makes most sense for your story.