People have a reasonable expectation of privacy. This means that the things they make public should be known to them to be made public. If I didn't intend to leave my DNA, then you shouldn't be able to use it unless you're investigating a related crime.
If I accidentally left my credit card on the ground, you would not be legally allowed to use it. I would have to have left it there on purpose.
The 'reasonable' is not about acquisition of information, but about the knowledge of the source. For instance, it is unreasonable for me to expect privacy in the trash bin, but not unreasonable for me to expect privacy in the bed of my truck.
If I accidentally left my credit card on the ground, you would not be legally allowed to use it. I would have to have left it there on purpose.
The 'reasonable' is not about acquisition of information, but about the knowledge of the source. For instance, it is unreasonable for me to expect privacy in the trash bin, but not unreasonable for me to expect privacy in the bed of my truck.