Mongo is installing to directories defined by its major version on Windows.
https://github.com/mongodb/mongo/blob/master/src/mongo/installer/msi/wxs/Installer.wxs

For the benefit of other installers that depend on mongo and need to execute command line procedures, it would be helpful if the installation directory of the currently installed version was accessible from a registry key. For example:
HKLM\SOFTWARE\MongoDB\Server\CurrentVersion with a registry value named "Path" with value "HKEY_LOCAL_MACHINE\SOFTWARE\MongoDB\Server\3.0"