Heck, I suffer from insomnia on a regular basis, sometimes I take a walk at 1 am with my dog. My DH knows I do this, and figure if the dog and leash are missing he knows what is up. I have tried to leave notes for him, but even with a designated "leave note here" location, he can't find them. I have also left without a note because I have gotten a cause of the "gosh, beef jerky sure sounds good". I am usually back within 10-15 minutes.
If I were leaving the house for something other than the above, I would leave a note and have my phone on. But, if it is a one time thing, I can see being upset but I would (hopefully) get over it quickly enough.
DH would leave a note, but only because he knows I am a worry-wart, and that I wakeup throughout the night, and would know he is gone.