... To add more mystery to this event there was no trace of a "slime Trail"
You'd be surprised how far they can jump, Richard. However, as is common for their species, they are not very quick to make a decision. They take a very long time to weigh up all the pros and cons of a situation before they act. Next time you see one in the garden, take the time to watch it carefully. If necessary, get yourself a comfortable chair, but don't take your eyes off it for 1 second, or you may miss it. Set aside several hours. It can be quite boring but if you do it in the front garden, you can also chat to passers-by, as long as you don't take your eyes off the snail. You could even invite them to join you.
... there are no known species of gastropod that can jump the 10 inches distance from the ground to the bottom of the door as shown. ...
Well of course, Richard, not from a standing start! They sprint and hurl themselves at their target. All over in the blink of an eye. Of course there is another possibility that you don't seem to have considered. You didn't overtake a delivery truck somewhere near a French restaurant, did you? (Sorry Cyf)