Hi,
rkhunter print warnings “grep: warning: stray \ before” due to an upgraded Grep to version >=3.8.
Ref. 176 Warning messages since grep has been upgraded to 3.8 and It’s Past Time To Stop Using egrep….
Upstream seems to be aware, but no new version except a proposed patch (see end of first link above) - how to implement the patch and/or report to openSUSE package maintainer (msmeissn) ?
Thanks @arvidjaar 
A more general issue have been reported 2022-09-04 and the current status is “RESOLVED WONTFIX”… Unsure if the error messages have any (significant) impact on the result of running the script, but will continue to use it.
This is entirely different issue.
Seems to be the same issue:
Andreas Stieger 2022-09-04 13:32:19 UTC
GNU grep from 3.8 emitting warnings when invoking egrep/fgrep
https://build.opensuse.org/request/show/1001097
> egrep and fgrep commands, deprecated since 2007, now warn that
they are obsolescent and should be replaced by grep -E / -F
We should look to eliminate egrep/fgrep from the distribution and replace all usages of egrep/fgrep with grep -E and grep -F, respectively.
At the moment dropping these items breaks the grep build itself, it's all over the place.
Also, the maintainer of a given app is almost always listed in YaST, at least after installing the app. I emailed the maintainer on a similar problem with rkhunter needing an update of some specific files.