I opened so many browser tabs! And I finally found a video on "constant functions" that shows an image similar to what you're asking about–That image changed, from a multiplication sign to a division sign, to plus sign, etc., based on the type of function being set as constant. This character seems to be automatically generated by the calculator as a sort of status reminder.
I strongly believe that the character you reference following the 3, 5, and 7 in your PDF is a white plus sign on a black background; in other words, it's a plus sign. I strongly suspect that what follows the 2 is the same character but poorly reproduced.
Thanks to you and to your braillist for asking!
The video I found is here: https://www.youtube.com/watch?v=ECNsabnKZaE