Take what I say with a grain of salt because only DNROI makes the rules and rulings but here's my take on it and how I'd handle it as the RM:
A centerfire competitor may not remove his holstered firearm and put in in a cart or backpack, etc. for transport. I realize there's no specific rule that that states that so let's assume a competitor does it. Now competitor moves to next stage and takes the holstered firearm out of the cart and puts it back on. In my mind a match DQ under rule 8.1.1.5 would be justification. I know the competitor didn't "drop" the firearm on the ground but they are technically retrieving it, unsupervised which is why I'd use 8.1.1.5 for the DQ. I will bring this up to DNROI's attention so something can be added to our rules in the next revision.