I can't find bb_admin.php

In your main folder where all the other .php files are.

NOTE, however, that for security reasons, it may have a different name (if it was setup by your webmaster, for example).

As mentioned by tom322, bb_admin.php should be located in your forum's main folder. If you cannot find a file by that name, it is likely that the file has been renamed, as recommended by the miniBB developers, for better security.
You can single out the renamed bb_admin.php by comparing the files on the web server to the ones listed in the official list of miniBB core files.
All best -
mb

The name of this file should be set under setup_options.php -> $bb_admin

Great question :) Something like - I don't know what I did yesterday :)

Why Googlebot tries to find this file? Rare ... but he searched for the file. An integration error from me?

Did you rename it from default?
Do you have ANY link visible to it in public?

I am mostly sure Google could only index the links, which are visible on public pages.
Search bot couldn't have idea about members-only links or hidden links.

Sure Paul, is renamed

No links to the admin access, except in these files, there is the variable from setup_options with the clear name

addon_pfchecker
addon_premod_msg
bb_func_vthread
index
templates/admin_...

And of course in the file itself ... But otherwise no links within my pages. I'm watching the further with the daily review of the log files. This I observe almost a month. Currently I have the file again renamed .. and wait for an new search.

Not sure where do you have it in bb_func_vthread or index?
By default, the link to the admin panel is inserted in templates/main_footer.html only, i.e. below of all pages.
It should not come in the clear HTML format, i.e. it should be the tag {$l_adminpanel_link}. Not sure why to replace such a straight thing to something else.
If you have performed your customization, most likely you inserted it in the place where everyone sees it.
bb_func_vthread or index may appear such a place.

But ... is not my Code ;-)

Line 327 in bb_func_vthread
$extra=1;
if ($logged_admin==1 and ... {$bb_admin}action=viewsubs ...

I was just looking in all forums files where all accessed $bb_admin

i have only $GLOBALS['l_adminpanel_link']; in my footer. In your sources i change almost nothing! And never the place from your variable {$bb_admin}.

What i meant Paul, i give only the variable $bb_admin in setup_options a name and set the l_adminpanel_link in my Footer. Thats all what i do. No specific changes or replaces in the source code.

This is a condition for the logged admin only :) It's not visible to a search engine.

It appears to be fine then.

So, what kind of the access string do you see in logs then? How does it look, could you copy it here (removing exact paths, of course)?

Good :)

But I have this access in my logfile, and for that i find simply no explanation:

Remember that any HTTP header could be easily faked. This could be a kind of "hacking" attempt, which look for possible placements of your admin file. BUT, if you renamed the file and the story appears, and the log shows exactly the same file as you put it on folder, let me know. Then it could be truly Google indexing attempts, and it needs more investigation.

Most likely scenario - you have/had Google adsense on the forum, right? Then you might have tested or left adsense code on one of the admin templates. That's why robots 'remembered' this URL and keeps coming back for some reason.

Thank you Tom! Yes ... you're right! On the Adminpage, these inserts are completely pointless- Will I be rectified immediately. Thank you for this helpful hint.