Small Flush on a Paired Board

Party Poker $50 No-Limit Hold'em, (9 handed) converter

MP2 ($48.50)
MP3 ($55.95)
CO ($40)
Button ($38.70)
SB ($60.35)
Hero ($31.45)
UTG ($54.20)
UTG+1 ($105.85)
MP1 ($26.60)

Preflop: Hero is BB with 5[img]/images/graemlins/diamond.gif[/img], 7[img]/images/graemlins/diamond.gif[/img]. CO posts a blind of $0.50. SB posts a blind of $0.25.
<font color="#CC3333">UTG raises to $1</font>, <font color="#666666">6 folds</font>, SB (poster) calls $0.75, Hero calls $0.50.

Flop: ($3.50) 8[img]/images/graemlins/spade.gif[/img], J[img]/images/graemlins/diamond.gif[/img], 4[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(3 players)</font>
SB checks, Hero checks, <font color="#CC3333">UTG bets $2</font>, SB calls $2, Hero calls $2.

Turn: ($9.50) 8[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(3 players)</font>
SB checks, Hero checks, <font color="#CC3333">UTG bets $10</font>, SB calls $10, Hero?

The SB's check call on the flop and turn is what sets off the alarm here... Thoughts?