I've never driven a Jeep without a cap, but in my 35+ years of driving I have had 2 brain farts and forgotten to replace the cap on other vehicles I owned. In both instances there was no doubt the cap was missing. Oil was evident on the hood and components.
Running without a cap won't hurt your engine (as far as mechanically, if you didn't blow out all the oil). It just usually makes a mess to clean up.
Makes me wonder if the place that changed your oil didn't lose it. Not that they did... just that it's an easy claim to blame the driver when they know nobody checks such things when they bring it in for service.
Bottom line. Your OK. Get another cap and install a new "idiot" light on your dash that says, "Do You Have Your Oil Cap?"