When I go to bed of an evening my dog comes up to bed with  me - only for a while though, either until I am asleep, or until he hears my husband come in, then he goes down and greets my husband then gets into his own bed.  I also have two grown up sons, but it doesn't usually make any difference if they are in or not - his ritual is generally the same.  Anyway the point is, last weekend my husband was away for 3 nights (also my boys were out all night for 2 of these nights), on the first night I was home alone (with my dog), I went to bed, up he came, walked around the bed but then went straight back down and got into his own bed.  Also the same on Saturday evening, we were home alone again.  On Sunday evening my youngest son was home, so when I went to bed he came up to bed with me and stayed up with me and then went back down to his own bed when I was asleep.  Last night my husband was home and he done his normal, which is up to bed with me, then when I must be asleep he goes back to his own bed.  This is so weird, I don't know whether he was waiting for my husband to come in, or whether he felt the need to stay downstairs!  I couldn't understand why the two nights we were on our own, he decided to stay downstairs!

I would expect that he feels most comfortable in his own bed/territory where he obviously has been well trained to go to his own bed, he obviously likes to escort you to bed and make sure that he knows where you are, I would expect on a protective stance he most probably feels better to do his job with stairs, back and front entrances easier to guard.

When hubby is not there he most probably took the view that he did not need to wait to get back to where he feels most comfortable, so went straight to his own bed, he may have felt a greater need to be where he feels he can protect better, there may also have been a part of him worried at why hubby was not home too.

From a human point of view we most probably would like the dog to stay longer with us, as we would feel better protected, but from his thought processing he can just retire to his own territory quicker than usual.

Bless him, at least you will feel very safe with him. :-)

He went to sleep downstairs because that is were his bed is...they can only really get a proper restful sleep if in there normal routine which for your boy is in his bed.
If you had brought his bed upstairs maybe he would of settled in your room with you- but it might just lead to confusion and worry as he normally sleeps downstairs. 
I live in the house on my own well with a child but I like my dogs to sleep downstairs then if anyone is wondering around outside my house they react must quicker . When in my bed a robber could come in and have a cuppa tea and my snoop would still be crashed out on my bed with me snoring his head off !! :-)

It sounds like the dog may have been responding to signals you were giving off. Perhaps he was taking on the role of guarding the house if he was picking up from you that something was different? If your husband typically gets home late, then I doubt the dog would know he was away, at least until the time when your husband normally gets home.

I agree they settle into thier routine where they would normally sleep .I find when doing  shows if we stay overnight in a hotel my girl wanders and cant settle ,wakes me in the night with a lick then sleeps all the way home in the car!