Wow .. I would be totally scared $hitless .... I'm assuming you have a gated backyard for you to leave the dogs out on the patio.
So you have to then decide if somebody took him, or he was hiding somewhere, or there is somehwere around the fenced area that he has managed to sneak in and out of. Either way... if you have a fence i would check it incredibly carefully, and then NEVER EVER leave your babies out by themselves againl
This is in no way judgemental, but I don't care if it's the most perfect weather conditions EVER and I'm ONLY leaving the house for 25 seconds ... every dog I own is in and accounted for!! NOBODY gets left behind ... because there are too many fricking lunatics out there.