Of course, all of these illegal searches were "voluntary" so there is no forth amendment violation. Kind of like a cop asking you to open your trunk. As long as it is posed as a question and you voluntarily comply they are in the clear. Of course refusal means that you have must something to hide so then the bullying and threats start.
Based on my LEO training with the local prosecutor (Oregon) I can offer the following advice, keeping in mind that I am not now, nor have I ever been a lawyer. If the search is voluntary, it is not by definition illegal.
Some police officers try to turn every traffic stop into a drug case. I didn't do that unless it was a profile stop. (Nothing racial, but a real profile stop is someone leaving a location where drug dealing is known to go on.) Find a real reason to make a traffic stop, get license, registration, etc and deal with the traffic violation. Give a verbal warning for the traffic infraction, return documents and say to the driver, "you are free to go."
Immediately engage the driver in casual conversation. Something like; “Mr. Jones, I noticed that you just drove away from that green house on the corner. How well do you know the person (name if known) who lives there? Are you aware that the person who lives at that house is known to be a drug dealer?
You aren’t a drug dealer, are you? No, I didn’t think you were.
I hope you can understand why I would be suspicious of that. Do you have any drugs on you, or in your car?
No? Just to satisfy my curiosity, do you mind if I look in your car?
(It’s amazing how many people who are holding dope allow a consent search.)
This is where it gets critical. Some officers may “bully” or lean on you to allow a search, but you have the absolute right to refuse. Ask; “Am I free to leave?” (If you are not free to leave, you are under arrest and visiting a suspicious house is not probable cause to arrest.)
Sometimes there is a legitimate reason to profile drug customers, or mules. The whole idea is for the word to get back to the dealer that the cops are watching, so they will feel the heat and set up business elsewhere. As with any other law enforcement tool, some officers will get over zealous and abuse it.
Keep in mind, a consent search can be withdrawn at any time.