Depends. I've seen cops seize automobiles and all property in them during "routine" drunk driving stops. Then again, when the officer asks the guy to STEP out of the car and he FALLS out, instead...mayhap it's a good idea to get him off the road :o)
And a firearms violation ain't exactly "routine."
I understand you are attempting to dance around this incident, however, being stoped while DUI is not a routine traffic stop for a moving violation, unsafe lane change etc........
And no, they do not seize your belonging for a routine traffic stop, no matter how hard you dance....... Not yet anyway...