So, I came up with an idea, and after some research, I've identified someone doing it already and they own a patent. My idea has some variances, but they are similar. I'd like to avoid disclosing the exact idea, so I'll speak in generalities. They are using sensors to measure sound and temperature to detect the occurrence of an event. If I use sensors to measure sound, temperature, and smell as a differentiator, am I able to patent this and use it without worrying about patent infringement?
You getting a patent and you avoiding infringement are two completely different things.
People get a patent by having something that is conceptually different from what has already been done (or described in a published document) and not obvious in light of what has already been done or published. Getting a patent does not give you right to make something; it gives the right to try to stop somebody else from making something.
To infringe a patent one needs to make, sell, import, etc. something that falls under the definition of a patent as expressed in its claims. A very short answer is if your invention does not need an element that is set out in a claim, you don't infringe. If you literally "add-on" an element to something that is patented, presumably your product has all of the elements of the patented item and therefor infringes. Sometimes an improved product can only be sold with a license from the patent owner of the original item.