dp and I live in town A (not together). His ex lives with their dcs in town B which is 2 hours away by car. We have what I would have described as a very easy going, happy relationship. Or so I thought!
a week or so ago, accidentally, dp sent me a message for his ex confirming he was picking up the dcs and taking them out on the evening of day1 (he usually stays there as he has a room at their house). We already had plans to meet on day 2 and he was staying at mine and spending day 3 together.
a few days later, he sends me a message saying he is seeing dc on the evening of day 2 and 3 and can we rearrange. I said fine (and thought him and ex must have just rearranged day 1) and he said why don't we do lunch on day 2 before he goes to town B for the evening.
coincidentally for work, I happen to be driving through town B on day 1. I stop at a petrol station and am filling up when lo and behold, I spot dp's car driving by. He has no reason to be in town B at all. I was going to call him then I thought let me just drive past his ex's house and see and lo and behold, his car is there.
day 2 he was going to meet me for lunch (as arranged) and I was going to be coming back from somewhere up north and arriving at Euston. I predict, in my head, that he will now come up with an excuse as he normally spends the whole day with his dcs when he goes down the night before. Lo and behold, morning of day 2, he says sorry he can't make it as he's ill
and says I'm really sad because I wanted to surprise you at Euston (he knew what train I was coming in on). I didn't say anything and he then sends me a picture of an online booked train ticket from his train station to Euston which would have arrived 45 mins after my train arrived 
I am not going to confront him till we see each other person but why on earth would you lie like this? I know there is nothing between him and ex but if this was innocent, you wouldn't lie like this would you? I suspect he is covering for whatever he did on the night of day 2 :( (as he would have come back from ex/dcs on day2). Should I just confront him with this or just tell him I think something is going on?