If the PI will give you peace of mind, you could try it. If you do, and it turns out he is faithful, and he moves back in, will you ever tell him about the PI?

If I did hire PI, No. Not sure what purpose it would serve to tell him.

Yea, I did, paid 5K cash // got nothing out of it. of course it was a male PI following up on my male spouse!! >> ha ha (dirty D's have a way)!!

The only reason I wondered about telling him was for transparency.

You deserve either the peace of mind of an "all clear" or the confirmation that something IS going on. Cheaters have lost the right to privacy IMHO the minute they stepped out of bounds. Your well being is THE most important thing right now.

I agree with betrayed, and I agree with you, there's no reason to tell him. He cheated and lied to you, so he's set the bar for transparency, and a cheater who expects such, is out of touch with reality.

As long as you have access to his digital devices, including his phone, and email accounts, there's no reason you can't keep tabs on his activities until you feel safe enough to build some trust. Also, where does he go that you feel the need to keep tabs on him? Isn't he coming to your home after work, or calling you to go out for dinner, or better yet, inviting you over to his place to cook you dinner? Let him prove to you physically, that he's changed. Words are cheap, and so is holding hands, and going to the movies. This doesn't show he's changed, this just shows he knows what to do to please you, and there's a huge difference, and I think you're being fooled by his simple actions.

If you've been apart for a year, you need to handle this as if you were starting anew, and dating seriously. Most seriously dating couples end up living together practically, where they spend most or all of their free time at one another's place. I think this would be ideal, and probably your best chance at seeing if he walks the talk, before investing in living together. You're doing this because you're afraid he'll stray if he's not under your roof, and that's the worst reason to take someone back into your home and into your life.

I'm still on the fence about the PI. It's funny what mistrust can do to a person. I get suspicious when he's too nice, too sweet doing all the right things, I'm also suspicious when he is distant, vague. It's a no win proposition and I'm going to up my therapy visits to explore this. What this has told me is I'm not ready to live with him again right now. I may still do the PI thing but not quite yet. My own sleuthing has turned up nothing - he goes where he says he will, he's home when he says he is, I can't find any evidence they are seeing each other (boy have I tried)! So if he is a changed man what I know now is I'm a changed woman but not in a good way. If he is trustworthy but I can't ever trust him that's a miserable life I'm setting up for myself. Lots to ponder here.

^ Good plan! And he's going to be on his best behavior now, give it time then hire the PI.
But yeah, give him time to see if he walks the talk, bringing him under your roof now would probably be picking up right where you left off. And if he's sincere, he WILL be there in 6 months, in a year, no? If he really loves you and wants you, he will wait. I would also expect him to have identified what made him cheat and what he will do should those triggers present themselves to him again.