Was a speech bubble there when it was being serialised in Le Petit Vingtieme?
If it is the first panel on page two that you mean, then yes, good spot.
The panel is redrawn in the book, and the original frame is widened, with the book-case being an addition to the colour version.
Originally there were two speech-bubbles: one for Tintin, at the top of the frame, where he said "Monsieur le Professeur", and then the professor's dialogue followed beneath. The two balloons overlapped, which is a bit clunky, so they obviously took the decision that Tintin's speech was superfluous and deleted it, and the professor's balloon was redrawn in a new position.
However, for some reason that seems odd to us now, it looks like the tail of the original speech bubble was left in place, and even inked in.Update:
After some additional thought... My sneaking suspicion
(it is an absolute guess!) is that the alterations made to the frame, during the re-drawing and colouring process, included a patch placed over the old bubble to cover it up, and the bit placed over the tail became detached at some time between the copies being made for the colourist(s) to do their work on, and the revised line-art going in front of the camera to be reproduced for print.
It wasn't unknown in the days of cow-gum for pieces of artwork to become unstuck if the glue dried out, and such a small thing could easily go un-noticed.
It would at least explain why the colour-work (showing books) and black-line (where the tail is included but outlines for the books "behind" it aren't) don't match...