The Quick Filter and Search functionality in Thunderbird does not look at the BCC field. It should.
It means that if I am filtering sent mail for a recipient, messages that were BCC'd do not come up.
You can do a search of BCC using the Edit --> Find --> Search Messages (Commad/Ctrl-Shift-F) but that is cumbersome and it makes more sense for it to just be included in searching recipients when in sent mail (or when doing a global search which finds some sent messages and not others).
I have just upgraded from TB 68 to 115 and now 128 and I was hopeful this would have been addressed, but, alas, it has not.