Yes, you should be upset.  They are divorced, their children are adults.  There is zero reason for them to speak at all..... ever.

It sounds like he wants to have two ex wives. 

Why does he need to visit his ex wife's new home?

My husband hasn't even talked to his ex wife in 12 years, much less been to her home.

What was his reason for visiting?

Just ask him why he did it and take it easy yes ya have the right to get upset 

It depends on their relationship and most importantly how you feel! My best friend is still friends with her exH and they don't have any kids together. In fact her now DH, exH and exH gf all hang out from time to time.

But like I said it's about your feelings and if your DH visiting his ex crosses boundaries for you it's something you should tell him.

For your DH to go to ex-wife's new house or old house.  If the kids were young and he wanted to see how they were situated there, I could maybe buy that.  But if the kids are adults?  No way.

If it's not something the two of you comfortably do together then you have a right to be uncomfortable. I would tell him to "pick a house." Kids are grown - there is not need for communication or contact. The kiddos can be the go betweens for holidays or other events. 

It depends on how much of a conniving twat BM is.  If she can be a friend to both of you, that's fine.  But if she's unfriendly to you, no.

I can trust my DH to never cheat on me but Satan is disrespectful of me and our relationship.  Neither of us associates with people who disrespect our partners.

My ex DH used to "visit" BM's house . Ex SD was a young adult. I turfed him he ended up going back to Donut Queen BM. 

If your gut tells you LISTEN.