Op, this isn't good.
However difficult today is going to be please do not ask him because he's going to deny it.
You have the upper hand here (of course it doesn't feel like that right now but not many people get the opportunity to know beforehand where and when he's going to be meeting someone).
So let's look at the facts..
He doesn't usually work overtime but has taken on a shift when it's not necessarily needed (financially) when your family, that he likes, has a planned birthday meal and gathering.
He also isn't sure on when he'll finish said shift as there's lots of work on.
He keeps his phone attached to him, on silent and password locked.
A mans name who you've never heard of is planning on meeting him either today or in the next few days.
A man doesn't usually care what he's drinking when meeting up with lads.
Your DP is driving 40 mins away to meet this friend, in a bar that has a hotel attached to it.
I think it's very very likely that the meeting is tonight.
You have multiple options and many that are easily done that can build enough evidence to either prove or disprove this awful potential affair he's having.
You can phone his work for whatever reason to check he is there. Do not do this between 3-6pm as he may stay on knowing he's got to kill that time. Is there anywhere he can get ready, changed, showered there?? Check his wardrobe for his favourite shirt/ jeans/ aftershave today..
Arrive at the said bar at 7pm (6:30pm is too early in case one is late) or get someone else to do it for you. A friend that you would trust.
Ring the hotel. Just say 'Hi, please can you check the reservation under the name of DP is a double room as my partner booked us a room but he mentioned it could have said twin on the confirmation email which isn't ideal for us'. They'll have no problem checking out if a room under the name has been reserved if you've given the details. If it's under her name and nothing comes up then they'll say sorry there isn't a room booked under that name and you apologise and say you realise you've telephoned the wrong premier inn.
I really wouldn't stay at your sisters tonight unless you take the steps mentioned above.
It's certainly not jumping to conclusions. There's enough red flags here to know that something isn't right.
I'm sorry you've been put into this position again. How awful that you've been here before, and whilst you were being induced?! 😳
Keep talking to us today. Try to keep a lid on it before you know either way though. Once he knows you've seen that message you'll be the bad guy for accusing him of something that he wouldn't do and the venue will be changed.