To me it feels like this lunch and this meeting a colleague are a way of testing the waters with you to see if your antenna pings. If it doesn't, and you cheerily wave him off to his work commitments, he's got the green light to meet someone in person he shouldn't be meeting with. If you look wary or suspicious, he cancels. That's twice now recently, isn't it? He's sounding you out on the most plausible scenario that he thinks you'll believe for cover of an illicit meeting.
Rightly, you are suspicious. After all, he's got form. There's something that caused you to look her up again. (may not even be here in the frame this time) but there's something fishy - lets face it, it's your third time at this shite so you are an old hand at the signals at this stage.
So think out the scenarios here - if he's cheating, or planning to cheat, what's YOUR plan?
If it's a telling off and then back to normal, honestly why rock the boat for yourself? Just tell yourself that he is playing away again and you no longer give a fuck and in fact, if the opportunity presents itself to you, you might even take a leaf out of his book and find an extra-marital dalliance. And just detach emotionally and just have one of those marriages. Because he's not going to change, is he?
If it's third strike and out for you, honestly who cares who he's talking to or what /when he's meeting them and what they are doing - just put the plans in motion to end the marriage. It is actually enough to say "Hang on, this feels like the thing that happened twice before, and I have no intention of putting myself through the wringer a third time so I'm off"
You don't need there to be another woman or absolute proof to end the marriage. The fact that you gave him the chance of trust at great trust to you some time ago and you no longer feel that trust any more, is enough.