Who can you call to check on odd smells?
Who you gonna call? Ok, I'll admit that Ghostbusters is low on my list of choices.

One possibility is to look for water damage somewhere. This will manifest itself in the form of mildew, mold. Perhaps water is getting in through your roof, or a problem with a window, etc. You might even have leak in the plumbing. (For example, we had a contractor redo a bathroom, but they knew nothing about plumbing, leaving behind 6 leaks in the walls.) To track that down, a pinless moisture meter is a relatively inexpensive idea here to locate wet places in the walls coming in. As a woodturner I have one, so it was a useful tool that i already had. Another key is to look for soft spots. Are there black spots appearing on interior walls?

There are specialists who can look for mold problems, but another good possibility is to look for something dead and decomposing in the walls or the ceiling. I'd bet on something large enough to smell, perhaps a raccoon. Second choice might be a family of squirrels or other small animals. Had you heard noises in the walls or ceilings? Those things that go bump in the night are animals.

In any event, there are animal removal specialists. Find a GOOD one. They can figure out where it is and how it got in, and if they know what they are doing, can even arrange to prevent such an event from recurring.