hi and 'merci' for your french hug :)
I use jquery gmap3 plugin in bp-checkins, their documentation might be useful http://gmap3.net/en/catalog/9-map/defaults-34 or http://gmap3.net/en/catalog/9-map/map-32
The default value zoom set is defined at various places so i think the best is to use a text editor that allows you to search in multiple files such as sublime text and search for 'zoom: 16'.
I've just tried this and it gives me :
/includes/bp-checkins-functions.php line 417
/includes/bp-checkins-shared.php line 210
/js/bp-checkins-dir.js line 80 and 137
/js/bp-checkins.js line 64 and 99
Adding a hook to customize the map is an interesting idea, i'll keep this in mind for a next upgrade. So far all is in js.
hope it'll help you to achieve your goal ;)