It was mentioned earlier that there is nothing to fear if you're doing everything legally. That is not true. If you are doing things legally, you can still have your property seized if you are searched, legally or illegally. Usually it's up to you to prove that the search was illegal, which costs you money. You ask not to say anything if you don't have experience. I do. Firsthand. Back in Missouri, nothing ever happened to me because no one really cares what you do there. Here in California, however, it has been a different issue.
I can't afford to live in a nice part of town; I just moved to a different town, but while I was in the bay area, I lived on the Berkeley Oakland border. This is a pretty crummy area of town. My car is old and sort of beat up, plust it's a Caprice, so it looks like a gang-banger's car. I didn't have a driveway or off-street parking, so I had to park it in the street. Imagine my feeling when I walked back home one evening to find my car gone. I thought it had been stolen at first, but when I called the police, they said that my car had been towed. I asked them why, they had no answer. They towed it late on a Friday afternoon, too, so there was no chance I could call and ask them why. I spent the entire weekend trying to get my car back. I was finally told that my car had been towed because the license plates had expired. I was shocked--my plates were current through 2007 and had the stickers on them to prove it. Then they wouldn't release my car to me until I showed them the current registration. I told them the registration was in the car, which was impounded. After a lot of cajoling and much calling and frustration, I was finally allowed in to get my registration out of the glove box. I was flabberghasted to find that my car was not only unlocked in the lot, but it had also been tossed (searched). The contents of my glove box were all over the front seat and floor of the car, my ashtray had been dumped all over the car, and to top it all off, my registration was sitting on top of the pile, meaning that they had DEFINITELY seen it! Upon closer inspection, the Berkeley PD had left a screwdriver in my car that they had been using to take apart the panels under my dash (which were off when I got there). When I asked about this, they told me that they were allowed to 'inventory' the car for valuable items. This is simply a legal method for them to search your car. Sorry, inventoring my car does not include tearing it apart. Then, I showed them my registration. They then said that they didn't know for sure that the license plates of my car had the stickers on them, so I had to go back to the impound, take pictures of my plates and print them out to show that the stickers were indeed on my car. Only then did they release my car to me. It was infuriating. The whole reason they searched my car was because they thought it was a gang-banger's car and that it might contain drugs. I did nothing illegal and had nothing illegal in the car, but I had it illegally towed and searched anyway. On top of that, they were still going to charge me $250 for towing and storage until I pulled out a tape recorder. Then they were very nice and had a nice sergeant come and waive my fees.
So, yes, if you're doing nothing illegal, you might be OK in the long run, but you'll definitely have a huge headache, and a waste of time and money. In some states, if they confiscate material, even if it is legal to have, such as your red P, they may not have to give it back if it can be used to make drugs. You should be very careful with it. Some cops are cool, others, like those here in Kalifornia, are not. In Kalifornia, if they police take a firearm that is legally owned by you, for any reason--even just to test it or as part of search, legal or not, you have to petition the state to give you your gun back--and they don't have to give it to you!
Also, there is the federal drug statute. Say your friend is riding in your car and he has a dime bag of weed on him. If the cops pull you over for a routing traffic stop and happen to find weed on your friend, they can seize your car. Even if the search is thrown out, they are not obligated to give you your property back--you have to sue them to get it back, and you'll only get it back if the search was illegal--even if your friend didn't tell you about the drugs and you had nothing to do with them.