AQ

[crockett]

UTG+2 is unknown.
Standard?

Party Poker 1/2 Hold'em (10 handed) converter (http://www.selachian.com/tools/bisonconverter/hhconverter.cgi)

Preflop: Hero is BB with Q/images/graemlins/diamond.gif, A/images/graemlins/club.gif. MP2 posts a blind of $1.5.
UTG folds, UTG+1 folds, UTG+2 calls, MP1 folds, MP2 (poster) checks, MP3 calls, CO folds, Button folds, SB completes, <font color="#CC3333">Hero raises</font>, UTG+2 calls, MP2 calls, MP3 folds, SB calls.

Flop: (9.50 SB) 5/images/graemlins/heart.gif, T/images/graemlins/club.gif, 9/images/graemlins/diamond.gif <font color="#0000FF">(4 players)</font>
SB checks, <font color="#CC3333">Hero bets</font>, UTG+2 calls, MP2 calls, SB calls.

Turn: (6.75 BB) Q/images/graemlins/spade.gif <font color="#0000FF">(4 players)</font>
SB checks, <font color="#CC3333">Hero bets</font>, <font color="#CC3333">UTG+2 raises</font>, MP2 folds, SB folds, Hero.....

[TwoShedsJackson]

Clearly you need to worry about KJ but a weaker Q being played aggressively is likely - absent a read I'd call his raise and check/call a blank on the river.

I wonder is it better to treat unknowns as incompetent or competent players?

[Bez]

I'd be tempted to re-raise given that your opponent may have QJ or KQ, but I think this would be too costly when your opponent had the straight, especially since those hands you're ahead of have a redraw. Call down.