So for me to be clear: the core of the issue is that - based on limited evidence - you believe that your boyfriend may still be seeing his ex?

As odd as it may be, I’d suggest having some sort of dialogue with ‘Jane’, get her side of the story. It could be that she’s saying this out of spite, as some bunny-boiler method to “win him back”. On the other hand, if she has receipts (as I’m told the youth say), that complicated things.

After talking to Jane - and if you’re still having doubts - time for a very direct conversation with your boyfriend as to what’s going on, why his ex is still creeping around, and any useful info gleaned from Jane.

Good luck! Hopefully this is just an ex who can’t let go.