Fyi and fwiw, while I didn't find a "convenient" way to rework the session decoding issue, I am incorporating the use of the WhosOnline class that was brought into ZC 1.5.7 such that if it exists it overrides the former process... What this means is that presuming that the old method worked for older ZC versions, the new code will still work there as well. BUT, if one forgets to include or chooses to exclude that class, then there are situations where the admin will get logged out when viewing the User Tracking page.

Swguy, I also have moved the delete "process" to a method within the observer class so that it may be called from outside of the main body of the User Tracking admin panel. I would like to consider making it part of the "routine" daily check such as with new, featured, sale like product. At least that way the load is low in "routine" site usage rather than on each page load on either the catalog or admin sides. Such an option though does make it a little more difficult to "control" who can make that possible, but its something that has been made more available.

Just uploaded version 1.5.10 for review:
Code:
Updated 01/02/2020 Version 1.5.10 mc12345678:
1. Addressed strict error reporting associated with admin_id as an array key.
2. Verified operational in ZC 1.5.7. up to PHP 7.3.
3. Revised the display of session data so that the date/time was not wrapped and allowed the URI to be displayed over a longer row.
4. Added javascript to bring the selected session to view.
5. Began using array variable to simplify consistent modification of page html.
6. Added ability to disable display of admin activity.
7. Refactored the observer class. Eliminated excess else statements.
8. Moved code to a variable assignment instead of processing within a function.
9. Made use of some of the unused internal variables.
10. Removed redundant checks against the value of some global variables.
11. Added database removal code to observer so can be used more robustly, even possibly automatically.
12. Simplified zen_check_bot function.
13. Updated the installer to remove variable $current_version, ensure operation,
14. provide compatibility for longer version identifiers,
Download location will be at/near: User Tracking - Zen Cart Plugins (zen-cart.com)