First escape. (Nightmare).

[Bogertophis]

...Is it possible that he’d gotten/caught something whilst he was down on the floor (carpet)? ...

You are asking if he might have "caught a cold" while on the loose? I wouldn't worry about it. Germs cause illnesses, not cool temperatures per se, & I don't think his
immune system was depressed for long enough to do any harm.

You're lucky you were right there & caught him quickly though, & I hope you remember that snakes learn their way around...he will likely remember how he got loose this
time & will try it again. How did he get out of the bag, anyway? Whenever I bag my snakes (like when I've moved cross-country, or taken them to programs), the way I secure them is that I first use a twist-wire around the loose end (make sure the snake is well out of the way), and then take that part of the bag & fold it over onto itself, & wrap a strong rubber band tightly around that...I've never had a snake escape, & I find this is also far easier to undo than actually tying knots in the fabric bags.

Snakes often move very quickly when they sense they're escaping...especially because it scares them even more to be pursued (even by those they "know").
It's just instinctive. One thing, if a snake starts going into a crevice or hole, it's important to never pull them out...you need to restrain them from going forward & hope they take the hint and back up, but if you pull on them with much force, you can dislocate all sorts of bones or organs, not realizing your own strength. Actually, it's just better to prevent the whole escape. Was there a hole in the bag?