Odds on proving softplay

[SimonG]

If you played in a casino out of town and you got down to two tables of 5 playing for the bubble, how many times would it take to be statistically significant for 3 players to fail to put a voluntary chip in the pot when against each other, but the other two players (from out of town)had their blinds raised each and every time?

I noticed the pattern after the second round and cried foul when 2 strong hands got checked down against each other.

But how soon could you prove to the management that something fishy is going on based on probability?

Any ideas - thanks!