Photo of the coop would be helpful, but they can fit into tiny spots, work latches, etc. Not likely to have locked everything back in place when he left though, so there's likely an access point somewhere. I disagree with it being a dog/fox/coyote/puma - likely everything you owned would have been killed - also not likely to have gotten into an enclosed coop without you noticing...
A betting person would go with a possum or raccoon. Other possibilities of course, but I'd start with those 2.
Also rare, but small chance a possum was inside waiting for your chickens to come in. Summer has late nights and it's not uncommon to find them out in daylight, later in the day, especially when still caring for their season's young. Someone was telling me recently actually that they went out and opened their coop and went face-to-face with a young possum that took a minute before it knew to run.
Local animal control can sometimes loan you a trap - maybe someone at
TSC or such will also. You can grab one new for around $40, or try craigslist or such. Dig a small hole in the ground, lay an open can of tuna or sardines in the hole then set the trap down on top of the hole - they'll go inside looking for the fish (and can't get to it so no bait stealing) and trigger the trap. Once they're at that stage, dispatching them is quick and easy...
