If you see an open gate this is the risk you take, that's just how it goes. What do you mean the Slesse trail is a designated recreation trail. Isn't that road for only for accessing the climbers descent trail from Slesse?, and the Slesse memorial trail is further up Chilliwack Lake road? I know of a climbing party that got locked in behind that gate this summer as well.
No there are signs for the trail at the 1 km mark so anyone driving in would be aware of the trail. Then, at the trail head there is a big brown post with painted white lettering on it for Slesse Trail. There are two routes for Slesse.