[BluffTHIS!]

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My guess is:

Case 5 for UTG, SB is slow-rolling 7-7 and MP1 took a card off with straight flush draw or an overpair.

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This is usually going to be the correct read. But let's also look at how the player with 77 is playing his hand. Slowplaying on the flop in an unraised pot is OK because the value of bets collected from other players is really worth more than the pot at that point. And although the OP's question is about his turn move and he doesn't give the actual turn action, the player with 77 now must pounce. If he gives 3 other players, who might have overpairs to make a better full or draws to a straight flush, a cheaper turn here, then he is asking to not make much if anything on the river while giving himself a chance to get broke.

As to Hero's question about the turn, I would fold with 3 other players having shown interest in that pot, even if it were raised and thus bigger.