There have always been stories, rumors and even scientific literature of dogs that can sniff out all sorts of ailments, including cancer. Some cancers cause changes in the body that could feasibly be detected by other animals by altering what and how things are excreted from the body. For example, a bladder tumor may alter what is excreted in the urine, including causing blood to be mixed into the fluid and this could get the attention of another dog's nose. However, there are a lot of things that can cause changes to the urine that other dogs may find interesting that are not cancer, such as a urinary tract infection. In this way, one dog might spend a lot of time smelling another dog's lady parts, but it doesn't mean that it is because of cancer. It could be from any number of reasons, including just be really interested in lady parts!
For more reading, here is an interesting article on dogs being trained to sniff out ovarian cancer: http://well.blogs.nytimes.com/2014/09/10/training-dogs-to-sniff-out-cancer/?_r=0 (http://well.blogs.nytimes.com/2014/09...)
Here is a scientific article (British Medical Journal) demonstrating the possible role of dogs sniffing out bladder cancer:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC518893/ (http://www.ncbi.nlm.nih.gov/pmc/artic...)
If you are concerned about your Dachshund, maybe its time for a checkup with the vet.