My advice is DO NOT DO ANYTHING TO INDICATE YOU KNOW ANYTHING... yet. I might be too late for this. It would seem that he lies, wholesale, likely about a lot of things.
Enabler, I agree with you. I have kept so much to myself for this very reason. I still do.
What I am going to suggest is about 'truth management / reality management'.
There is a huge inclination to take evidence and show and tell, say "I found this, you're doing this". What tends to happen in my experience is that your pwBPD will just adapt their behaviour. It won't stop the behaviour, it will just mean that it takes another avenue.
Exactly! Once they know you know, it only makes them go deeper under ground with their activities. You are correct that the actual behaviour doesn't change though.
At the moment, you know about this mode of communication and you can continue absorbing information. Start your surveillance. Take note (and I mean literal notes) of where he says he is, what he's doing and who he's with. Try to tally together data all the while being conscious not to rouse his suspicions and alert him to your suspicions. Put 'evidence' back where you find it, take photos of things rather than keep them and always make sure that it's 100% safe to search before looking, I have been caught out a number of times when my W has returned because she'd forgotten something.
I hear you on all of the above. He has no idea that I have a secret second phone number that no one else knows about. I monitor from there. That way he can't see that I am online and he is more inclined to feel free to chat with the ex.
I used to keep a note book of things years ago. You know, so I could go back and see that yup he did say/do this and that and is now spinning an entirely different story. I guess it just got too tiring and time consuming and so I stopped. I agree it's time to start again.
Thank you for those suggestions.
