Yepp, most likely the guild guy ratted him out.
Also while it looked cool, did that thief actually wore the insignia of his theft guild on his shoe soles? (It was so attention-grabbing, I couldn't ignore it.) If that is true that's just too hilarious.
I trash-talked (commented) the manga quite a bit, but just to be clear: Up to now the setting/plothook of this manga and its execution isn't too bad. I somehow like the pacing, the drawing style. Though sometimes a bit predictable, because it abides to quite some typical isekai tropes and even sub-plots, I want to know what happens next. I also don't dislike MC at all. It's just there are quite some ridicoulus actions of the characters here and there and events, that haven't been thought through by the author. Then again, it IS fun pointing out those screw-ups.