Ok, heres another scenario.

The tap water doesnt kill the leeches, it just irritates them. Perhaps your leech was very small. He let go of your turtle and found his way to above the water line where he waited until the chlorine dissipated from the water, then he made his way back down to your turtle.

How deep did you fill the temp tank?

How long was your turtle in it?

And once again, perhaps your tap water isnt as chlorinated as mine.