Only Root has access to some of these locations so you have to login as root to capture the installers. Library/Updates/numbered folder beginning with zzzz031 or just 031 which may vary by OS version/ private/var/folders/zz/zyxvpxvq6csfxvn_n0000000000000/C/PKInstallSandboxManager/5AF5AE82-B23B…(numerical string which may or may not be unique).activeSandbox/Root/Applications/ There are newer locations of downloads which include: Things have changed with each OS version since this article.
There are several online articles on capturing app store installers from the app store.