I'd make sure to write her number down and go through the last couple years of phone bills to see whether her number shows up at other times and/or if it shows up with alarming regularity.
I wouldn't confront just yet - you don't have enough solid evidence and he'll surely find a way to dispel anything you might suspect.
Bide your time...collect your evidence.
My suggestion is to dig. Every dollar of effort will earn interest when you finally confront him. See if you can figure out the dive by bank statements or credit card receipts. Check phone logs, etc. If you can discover the dive, then go there. See if you can figure out who she is. Go in, get a cup of joe, be nice, get her name. You can even try to get her last name with some social jiving. She comes up and says, "I am Karen, and I will be your server." You come back I am "Karen...." and give a long pause for the last name. Then introduce yourself as Hadley Hemingway. Pay with cash and leave a big tip. Be nice, leave. Camp out and get the tags and description of her vehicle. Take some pics with your phone. I am not saying follow her though. Find out the zip codes in your area. The google search her name with that zip code and she if anything pops. If you find yourself in the area, swing by and check to see if her car is present. If so, take more pics. Go find the name of a brutal or well known lawyer in your town.
Go get all the prints developed on BIG glossy, black and white 5x7s or 8x11s. Put them in a manilla envelope. Go take out a large chunk of cash. If you can't bluff, dont say much. Just throw down the pictures, the text logs and press. If you can play poker, get your face on. Confront him with the facts and then say the PI came up with some good photos but the juicy ones will cost more. Be dramatic, be ticked but cool. The PI will take the cash and drop them off at the lawyers office as promised. If you don't think a lawyer will scare him, then just say they will be uploaded to social media for maximum impact.
Ok, why Hadley. Hadley Hemingway WAS Earnet Hemingway's wife and the name points toward an affair that did happen. It was detailed briefly in his novel "A Moveable Feast." I just think it funny she is a waitress and you may find some irony in using fiction to sort our your own facts
In your case, it COULD be nothing.. however the thing of him going to the same place drinking with buddies is not necessarily a good sign. Could be innocent, but it's also how some of these things start. The booze, and the familiarity, and guys encouraging/ reinforcing bad-guy behavior with each other.. when there's a young female around, probably acting flirty for the sake of the business. Bad combination. Do some more digging, or tell him you'd love to have lunch with him one day. If there's nothing to hide, then why not? Then you can get an idea of the situation.
After Perv left home and before DDay, he spent time doing some things for neighbors who had moved, taking care of their house and such.
Something that I noted that I had not when he lived at home was that he constantly mentioned the names of girls...this was a semi-large family with several girls there and for some reason he seemed to keep saying their names. It was very strange, to me.
He also speaks frequently-or used to-of our neighbor and will say her name periodically. It's so odd to suspect him at every turn now, but his actions and all the crap caused that.
Just wanted to make note of the good comment about "pay attention".
Another red flag in that regard is that Perv was looking at women out in public for our entire married life and it escalated really badly during false R, but I would wonder if he was looking in crowds for OW, as he cheated on me with her.
Good luck DaVille. I'm glad you are suspicious, it will surely keep you on your toes and I'm glad you aren't sounding naive or in a fog or anything about WH. Very daring to have lunch there with him, but a good point as well.
I confronted Perv on the phone because my nerves got the better of me and finally when the lies ended, I apparently screamed in his ear and he dropped the phone.
I wish you well.
A person is a person, no matter how small. -Dr. Suess