His determination
and efforts should be rewarded by letting him make his own decisions, and
if
he WANTS to go back to where he came from, he can get there himself,
right?

Yes, he can get back, and indeed, his efforts (he must have climbed
vertically on some pretty slippery rocks) are well respected :-)

The other ones don't seem to want to get away (yet ?), he/she did.

Of course, I could just play hardball ("Oi, I'm the boss here !") but I just
don't want to do that.

The only reason I am considering taking it back (if I can catch it) is the
food thing. But as said in another post, maybe it's found its own stash ?
We have seen him come up when we feed the koi yesterday BTW, and maybe he
can catch the sinking pellets we feed to the sturgeon ?

These animals have survived quite a few years now, and although I know they
are not exactly in their natural habitat, I'm wondering about letting their
"instinct" (bad word) lead them.

Theo