Steve seemed very interested in how the undercover cop's widow was able to afford various things (TV, Sky subscription etc).
When she went to his car she clocked the number of pill packets and must have thought something was off - is this leaving Steve open to a bit of blackmail maybe?
Did he leave his pills at her house? Seems unlikely given his reliance on them, but they were quite bulky boxes and he looked to be empty handed when he left.
I don't have any access to catch-up so I can't go back and check myself.
I agree with all the other posters saying that Kate is definitely undercover and is only pretending to be mates with Jo to get information.