Thank you that solved the issue.

To test I removed my admin IP so it would be tracked. On deleting the record this then highlighted another warning.

PHP Code:
[31-Jan-2021 00:40:26 UTC] Request URI: /ADMIN/index.php?cmd=user_tracking, IP address: xxx.xxx.xxx.xxx
#1 require(../ADMIN/user_tracking.php) called at [../ADMIN/index.php:11]
--> PHP Warning: Use of undefined constant TEXT_HAS_BEEN_DELETED - assumed 'TEXT_HAS_BEEN_DELETED' (this will throw an Error in a future version of PHP) in ../ADMIN/user_tracking.php on line 267. 
Line 267
PHP Code:
    $deleted_session_text = $_POST['delsession'] . TEXT_HAS_BEEN_DELETED . ' has been deleted. <br />'; 
Despite excluding my admin address it is now still being tracked. When I delete I get
77ca6d2efef563fca5f494ebf137698fTEXT_HAS_BEEN_DELETED has been deleted.
Also there doesn't seem to be a define for TEXT_HAS_BEEN_DELETED in the language file?