The answer to your question most likely is, "You don't wanna know."
I have dealt for the last 6 months with this exact issue with my exAH. He would take my car, and the dog (the only tip-off I might have had to when he arrived home) and literally drop off the planet. Sometimes for 8 hours, then 12, then 24. And, at its worst, 48. He would not answer calls, nor texts. I had no idea where he really was, because it was never where he claimed he would be.
No amount of your worry, nor concern, nor your attempts to get to the bottom of it will change the fact that this is unacceptable behavior on his part.
And regardless of the fact that it's unacceptable, it's not going to stop.
I have learned this the hard way.
My son is now 9mos old. His father has had regular, good, and fatherly interaction with him for maybe a week's worth of his short life. The rest of the time (and you will learn this about your husband too), even if he was here with us, he wasn't really HERE.
I have already filed for divorce. It's dragging on and taking far longer than I wanted it to, but I have done it. I have put aside the notion that any amount of my support will keep my husband clean and sober.
Please, please, if you cannot do this for your own sake right now, do it for your children. It is far better to try and salvage some semblance of a visitation schedule or to be able to honestlyt ell your children that you tried everything, but ultimately it didn't work out, than to keep dragging them through the cesspool of a marriage that your husband has caused you to dwell in.
PM me if you want. I understand completely what you're going through.