[W. Deranged]

I didn't mean to shout too much, but I think this sort of play actually could qualify as a "big leak."

A9s does have some high card strength, but it is very likely to be dominated.

Loose players, particularly one's who are passive post-flop, have very different standards between what they'll limp/cold-call with and what they'll raise with. It's part of the reason they suck. Even a loose player, unless a known maniac, who is raising UTG probably has a pretty decent hand. It is almost definitely one which is ahead of A9 on average. The suitedness of the hand changes nothing really, as you'll be playing a short pot anyway and won't be getting great odds to hit a big hand like a flush. Also note that even lose-aggressive types will often be raising UTG with hands that hurt yours. Loosies might get high PFR numbers by raising hands like ATo under-the-gun and so on. Also note that since the player is loose and passive, raising here is not likely to buy all that much post-flop fold equity, and hence you'll probably need to showdown the best hand to win here.

What I think you may have revealed here is that you are overdoing the "isolation" idea. It is true that "expert" pre-flop play involves looking for opportunities to play short-handed pots against looser/weaker/worse players. But you need to be very thoughtful about what kind of hand you have, what kind of hand your opponent is likely to have, and what kind of bad player they are. It can be great to isolate players with loose raising standards pre-flop by three-betting. On the other hand, it may well be negative EV to try to isolate players with loose calling standards but decent raising standards by three-betting. The difference is very important.