Rodents, aka Rats, are very common in crawl spaces. Inspectors will refer to them as "rodents" because it is oh so much more "euphemistic." (Plus it might be squirrels----aka, "bushy-tail rats")
People HATE rats (usually).
I have seen rats kill a deal on a house as quick as asbestos or mold.
One of the most common ways they get into a home is through damaged crawl space vents. Sometimes they make their own way through these vents. Other than attics, crawl spaces are the most common places to find signs of their being around. If you ever saw "The Secret of NYMH" you probably already have a sense of why they like crawl spaces---these places are NYMH! It is a good idea to routinely check these vents for signs of infestation. This picture is a very good example of a hole in the screen either made by the critters or made by other means. The dark staining on the wood under the hole is where the grease from their body hair is accumulating on the wood---a sign that they have been coming and going for quite some time.
Wiring should not be run through the screening of these vents because rats will eat the wire off, and then be able to get through the hole where the wire was. They will (if there is something they want to get to bad enough) chew through the screening or widen a hole already in the screen.
Hopefully they are not doing a sequel to "The Secret of NYMH" in your crawl space.
PS, for those of you that are new to my blog (or for some other "unexplained" reason have never noticed) all pictures and smiley-face inserts (emoticons) have messages that show up when you point at them with your cursor.
* * * * * * * * * * * * * * * * * * * * * * * * *
WA State, Home Inspector Advisory Licensing Board