Holding it doesn't create a good enough ground plane.
You have to mount the antenne to your vehicle properly.
If you want a rig that you can pull-off without dealing with leaving a mount, use a magnetic mount antenna for the CB. They aren't great, and you're FAR better off with a real mount, but if your priority is covertness, then it's the logical choice. One advantage to using the magnetic mount antenna is it already has all the cable and connectors on it. So you just plug the radio into power, plug the antenna's cable into the back of the radio and put the antenna on top of your car. The disadvantage is it will cut your performance WAY down. It's the trade off for protability.
Use a "y" adapter for your radio antenna. Leave the free end under the dash, then you can just put power to the scanner, and plug in the cable. I don't know as much about scanners honestly, but alot of locals use the trunking stuff, and there just may not be anyone to listen too on the scanner if it only scans frequencies no one uses anymore. Here in Virginia Beach they simlucast all the new fangled digital signals on the old scanner frequencies so you can still hear everything with an old style scanner. I haven't heard of too many other cities doing that though.