Slow plays at loose-passive tables? (LC?)

Longtime reader, first-time poster. No good reads on any player (and no idea how people saw me) but a generally loose-passive table. My apologies for having no idea how best to present this hand, since I'm actually interested in feedback on MP1's play.

Both MP1 and I made at least one seemingly obvious mistake, though MP1's is the more interesting one, especially if it's a clever play after all.

Party Poker 3/6 Hold'em (10 handed) FTR converter on zerodivide.cx

Preflop: Hero is BB
<font color="#666666">2 folds</font>, <font color="#CC3333">MP1 raises</font>, MP2 calls, MP3 calls, <font color="#666666">2 folds</font>, SB calls, Hero calls.

Flop: (10 SB) A[img]/images/graemlins/spade.gif[/img], Q[img]/images/graemlins/club.gif[/img], 7[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(5 players)</font>
SB checks, Hero checks, <font color="#CC3333">MP1 bets</font>, MP2 calls, MP3 calls, SB calls, Hero calls.

Turn: (7.50 BB) 5[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(5 players)</font>
SB checks, Hero checks, MP1 checks, MP2 checks, MP3 checks.

River: (7.50 BB) 5[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(5 players)</font>
SB checks, <font color="#CC3333">Hero bets</font>, <font color="#CC3333">MP1 raises</font>, <font color="#666666">2 folds</font>, <font color="#CC3333">Hero raises</font>, MP1 calls.

Results in white below: <font color="#FFFFFF">
Final Pot: 13.50 BB
Hero has Ks 5s (three of a kind, fives).
MP1 has Ac Qd (two pair, aces and queens).
Outcome: Hero wins 13.50 BB. </font>