In our school system, any time children leave the premises for any reason there must be a permission slip sent home and returned signed by a parent. Period.
Those slips include a section the teacher can check that says the field trip does or does include transportation, and if so, what kind of transportation (school bus or charter bus).
There would simply never, never be this situation you describe in our school system. I am amazed that the principal does not realize that the school could be legally liable if anything happens to any child at any time and the school did not get permission slips (which also in essence can be a waiver that protects the school!) in advance.
I disagree with your husband. Wait to see if it happens again? Seriously?
So he's OK with the idea that the school officials are totally blasé and casual about taking your child somewhere and you don't have a clue that he's not at school? Present your husband with the scenarios: There's a family emergency and you need to get your son, immediately. You drive to school...he's not there. There's an accident with the bus and the school isn't sure which parents to phone because there's no master list of kids who were on that bus because...there was no such list made, because it was all so very casual. Or forget the dire scenarios: The problem could be as simple as the bus goes to another location late in the day and ends up coming back to school late due to an accident or road works that fouled up traffic. So your husband is OK with arriving at school to pick up your son and realizing he's not there at all? Or with waiting for your son to come home on his regular school bus and then getting a call that you have to pick up your son because he's sitting in traffic somewhere?
The school says it does it for this one event, but if they do it for this, they are very likely doing it for other events. I would be as angry as you are. It is quite simply very bad policy and it sets up the school to be sued at some point by someone.
Unless you are OK with spending your child's elementary years wondering where he really is today, I would write a formal letter citing the chapter and verse of the regulations and asking the school to implement a policy of written permission forms going home before every trip off campus other than the "walking field trips." Those kids who do not come back with signed forms do not go on the trip--no ifs, ands or buts. That is the policy in our district. I know of kids who did not go on field trips because their parents didn't sign or the kid lost the form. Tough.
Be sure to send a copy of the letter to the school board as a whole and another copy to whoever is your area's school board representative (if you have an elected school board). Be sure the principal knows you have sent them copies, so he or she knows you have already alerted the school board to what is a violation of rules. Schools just cannot be this casual with the whereabouts of kids.
Oh, and you're going to get comments from parents or others about "Don't be THAT parent" or "We have never had any issues and we've done it that way forever" or "Teachers aren't going to like you if you pursue this." It's up to you how much those things will or won't influence you. But I can say that what the school did would be cause for reprimand in our system and (I would assume) in most systems.