What Reality said.
There is no 'trail' or 'trap'.
He said this hoping to flush out what you had found out about. The fact that he doesn't know what it is does suggest that there may be 'other things' to discover.
I am starting to reverse my more lenient view of him having heard this. Whether I am right or wrong, there is cruelty at play. If he hasn't set a trap, he is playing mind games with you (and hiding things from you) and if he has... well, same as above really.
This must be so so hard but in a few hours, you will have some answers. And it may work out ok. He might as my previous posts suggested, have a bit of a dark side that he has been to embarrased to share with you to date, and he might responded with the 'trail' story in a moment of embarassment and panic. You may discover that actually, his crime is to have fantasies over you to the point where he typed them out on a phone, and you may find this acceptable (once he has apologised for the ridiculous 'trail' red-herring).
Either way, I believe the 'trail' thing is a red herring - either designed to flush you out or just born out of panic.
My best advice now, is whatever comes out of your conversation tonight, think long and hard about whether you want to spend the rest of your life with his man. And if you do, fine but if you have any doubts, DO NOT GET MARRIED TO HIM. It will be hard to walk away from a wedding due to happen in two weeks time. People will be gossiping about why, what happened. Others including your partner - will try to channge your mind, you will feel guilty, wonder if you are throwing a chance of happiness away and your pregnancy will play on your mind.
BUT this will all be temporary.
Marriage is for life.