My HB came over to fix something ......from his GF house....REALLY weird! 

So, my STBX HB (36) has an 18 year old GF. He went straight to her place this evening after dropping my son off from parenting time. She lives about 15 mins from me.
About half hour after he dropped my son off, the firealarm in my house started beeping. Or maybe the carbon monoxide detector. Dont know. It kept beeping. I checked it and couldnt figure it out. My 16 year old son checked it. I was stumped.
I couldnt put the baby to bed bc the beeping was very loud and it was in the  hall near the bedrooms. What to do? I had no one to call.
I waited half hour trying to figure it out. Battery? Is there a fire? Dunno.
I did not know what to do. So, I bit the bullet and called my STBX Hb which is something I do not do. We only communicate as is necessary for the kids.
Tonight, he was at his GF house. We both knew it but I said nothing about it. Neither did he, but I could hear her in the background talking to her sister in spanish. 
I felt nothing as I asked him what to do about the alarm. He made some suggestions over the phone. None of them worked. He offered to come over. I was surprised but I accepted. 20 minutes later, he showed up and checked it out, adjusted a few things here and there. It worked. I thanked him. He said a quick bye to the baby and that was that. 
Hmmmm. It felt really weird. He's still my Hb. MY Hb has a GF. I asked my HB  to come over to the house and he came over from his GF house. Im the one who felt like I was intruding and imposing on them. Make sense? Im surprised he offered to come over too. I did not expect that. But, maybe it was his way of "thanking me" for not reaming him out about his being 1 hour late this morning for parenting with our baby.
It just felt really weird to have him come over under the circumstances.
Funny thing is, I had a pang of sentiment seeing him in he hallway - for like a second- I saw a ghost of him from 2 years ago fiixing the alarm in the hallway. We were happier then. I miss that ghost. But it was only a ghost. Its not him and it wasnt him today.
I see those ghosts from time to time. In the kitchen. The living room. Watching TV. Sometimes, they look right at me as if to remind me of their presence. Other times, they dont. But nevertheless, they fade away.....slowly...to black. And, then they are gone. 
What surprised me the most about this incident? I didnt feel jealous. I didnt feel mad. I didnt even feel rejected or hurt. I really didnt feel anything. Complacent? Numb? Just indifferent.
After all he has done, and after all the horrible words said between us - It is almost like I am at peace, finally. 
After all, his porn addiction, mood swings, cheating and all the other issues that plagued our marriage and destroyed it are not my problem anymore. I wash my hands of it.....of him.
Our only connection now is as parents. We are nothing else together. He is a complete stranger to me. I dont really expect much from him anymore. So, in a way, I kinda appreciated the fact that he did come over to fix the alarm. Im not reading into his motives and I know he did not do it for me, but maybe he did it for our sons sake. To make sure he's safe tonight- I think he wanted to check and made that effort. Im proud of him for that gesture. I dont know what to make of all of this.......yet.