My question , after looking at a youtube video on the Swagman Rack, is are you 100% sure you latched it? It looks like the latch system ratchets on and you have to release a lever to undo the latch to get the bike off. My question to you is where was that latch when you noticed the bike missing? Was it up/ down, where?
If it was up then maybe you didn't latch it and only thought you did. It happens.

If you are 100% sure you did and nobody took it off then I would think the rack/latch defective and never use it again unless I were to put a lock on the latch like they say you could.

Maybe your attention was on taking the Garmin off and you just forgot to latch it?

Whatever, I am sorry for the lose of your bike, a real bummer.

I have been following this thread because a long time ago I used to race Moto-x and one race I had the bikes on the trailer and got everything ready to roll. When I got to the race about two hours away I went to take trailer off and noticed I never latched the trailer to the ball. I did have the safety chains on though, I was lucky to be sure. If I had lost the trailer/bikes and somebody asked me if I latched it I would have sworn I did. Sometimes our attention is diverted for a second and we think we did something and don't. Just an opinion.