Just reminded of that Gandalf (wizard) hired Bilbo to the party of dwarves (all fighters? Balin would probably be the one who fits "cleric" best, being the wisest of the dwarves, having a high perception and fearless) for the position of "burglar".
I know the younger me was very confused as to why Gandalf would call his friend basically a criminal and how what amounted to what looked to me a upper class british gentleman could be skilled in breaking and entering.
"Burglar" would be someone who break in and take things. Being stealthy and skilled in entering places they're not supposed to be.
In a game of Shadowrun or modern/science fiction that would be the Infiltrator, intrusion specialist, Security breacher or whatever that has a much nicer sounding name, but to modern to be used in a D&D game.
You need something more old fashioned named, and "thief" simply fits.