Things to worry about when taking snakes outside: hawks (stay very close if the snake is crawling freely on the ground), climbing out of reach in a tree (don't let the snake get out of your reach), falling to the ground and disappearing into overgrowth (only place the snake on the ground in a clear area where it can't burrow under leaves or crawl into dense foliage), contamination with pesticides, fertilizer, herbicides, etc. (don't set the snake down unless you know the vegetation has not been treated with anything), overheating (do not take the snake out if the temp is over 82F, do not place in direct sunlight if it's hot, do not place on a hot surface) and aquatic predators (do not place in water where a frog or fish could eat the snake before you can react).
Things not to worry about: snake mites (snake mites do not live out in your yard- they live on other snakes, and in other snakes' bedding. You'd pick them up at a pet shop or reptile show or on a new acquisition, not out in the wild), snake running away (cornsnakes cannot move fast enough to outrun a human).
Be very careful when transporting your snake in a car. Snakes can disappear in a flash inside a vehicle, or rapidly overheat if left in a car in warm weather. I recommend placing the snake in a deli cup or snake bag, and placing that in a locking plastic container (with air holes) like the feeding container, Sterilite, etc. You can then place that in a reuseable grocery bag and be able to carry the snake anywhere discretely, so you don't leave it in a hot or cold vehicle.