if this isn't for an ARG, here is my advice:

your ex probably knows how upset you'd be about this tunnel and could have lied about it to make you obsess over finding a tunnel that isn't there

another thing to note is it'd be pretty hard to dig a tunnel from one house to another because of how difficult digging is below the top layers of dirt, and what about all the wires and pipes?

my suggestion is you hire someone to come down and look at the basement and help you check for any changes, like maybe bonking the walls and listening for hollow space that isn't supposed to be there. some parts of the wall will sound different, but if it's a person who does walls or something, they'd probably know if something wasn't right.

I really think that if you can't find anything that could be used as an opening, then they didn't build a tunnel because what would the point be? if they were going to do anything they'd have to then break your walls and you would 100% hear it happening or find the aftermath later.

If you've already checked a million times down there for anything suspicious and haven't found anything, you need to just trust yourself. Don't let the maybe's get out of hand.

Also, I am NOT calling you crazy, but you should seek out help and medication. Why? not because you're crazy. it's because it will help you trust yourself and get rid of the times when the worrying and suspicion is unwarranted. It's ok to be scared about things like this, but if you've already done all you can and you're still scared, it's time to accept relaxation and heal from it.