By first-hand experience and also by this other SO answer, usually IISExpress holds a directory with its own data files at: C:\Users\<username>\Documents\IISExpress.
There you can find the default directories for each web site log files (Logs\ and TraceLogFiles\) and especially you can find all web sites configurations in file config\applicationhost.config. That one has a <sites> section with a <site> node for each website created.
For each website, its <virtualDirectory> node specifies actual root location in physicalPath attribute:
<site name="YourWebSiteName" id="12345">
<application path="/" applicationPool="Clr4IntegratedAppPool">
<virtualDirectory path="/" physicalPath="D:\Physical\path\to\your\website\root"/>
</application>
...
</site>