Hi All,
I also posted a support request for this, and just in case it helps anyone in the future, I've included Matt's reply below:
Hi Toby,
Yes, providing the client has not been off site for any longer than a certain time (the session timeout length on your server) then the client should be returned and still logged in from before they left. There is no reason that WHMCS would log the client out, and nothing the gateway or module for the gateway has to do in order for that to happen, it's simply controlled by browser cookies/sessions. The only thing I can imagine might be happening here - if the time off site isn't long - would be a change of domain so you need to make sure that the exact domain entered in Setup > General Settings where the user will be returned to matches the domain that you are using to login from for your testing.
Regards,
Matt