What does this mean?
This is an HMRC-specific warning. The checker was unable to determine if a session timeout warning was set up.
Impact on users
If a session times out without any warning or option for the user to extend the session then it can mean that the user has to log in again, loses their place or even lose data they have entered so far.
How to fix
The user should be warned when their session is about to expire and provided with the opportunity to extend the session further.
The easiest way to accomplish this for HMRC services is to use the service timeout pattern.