M.,
If this had happened to me, then I probably would've made a joke about it and said "Oh come on, what's in there that you don't want me to see" and I would've walked right over and started going through the boxes and if he stopped me then I would've gotten mad and questioned him. Since you didn't really push him on it and the boxes are gone now you may never really know. All in all, I wouldn't make too big of a deal of it if you've been with him for 10yrs and have no reason to suspect anything. You might say to him though, "Hey, did you get those boxes taken care of at work?" Just see what he says and no matter how he responds just say "I kind of got the feeling that you didn't want me to see what was inside - what was in there?" And I have a communications degree...if he hesitates at all in his responds then he's lying. If he responds promptly with "old baseball cards or some photos that were old" whatever he says if its quick then I'd probably trust him. I hate to say this but guys are pretty simple minded so I mean, has he switched jobs ever? These might be boxes of old work stuff that he wanted to take to his current job to go through and then you're going to cause a fight over nothing. I have a couple boxes in the basement of old stuff from switching jobs, if I got a new job I'd probably want to sort through them and see if there was anything I could be using.
The other thing is - do you and your husband have any "sore spots" like an old girlfriend, or you hate that he is obsessed with football, or a family member of his that you don't get along with or an old friend. If there is anything like that between the two of you then he might have just found something that would've reminded you of that situation or person and to avoid a fight put the boxes in the car so he could dispose of them later - in which case if you end up fighting with him over it then he tried to avoid the fight and you end up looking like you overreact and he'll just say "See, this is why I didn't want you to see what was in the boxes". Women are more of talk it out communicators and often times Men are avoiders so this could be the case.
Anyway, I hope some of this was helpful to you - let us know how it turns out.
Take care!
K.