We know that guests who are in an attraction, or even in an attraction queue, cannot typically hear PA announcements. If a guest is in an attraction that they're allowed to be in, we can hardly expect for a PA announcement to be sufficient.
As for sign, yes, a lot of people have mentioned signs and posted closing times. The thing that we need to remember, though, is that there were also posted closing times for AK, HS, and EPCOT on that same day, and to the best of my knowledge, guests were allowed to make purchases in their respective gifts shops while making their way out of the parks after the posted closing times ...because that's obviously the daily practice. I've seen the Main Street gift shops remain open for shopping well over one hour past the posted closing time. If Disney went through the effort to make signs and post them around the park, it would not have cost them anymore to articulate that both attractions and shops would be closed, being that is the more unusual scenario from the norm. (The "usual" scenario being that shops remain open beyond the posted closing time.)
I would hope that a day guest exiting Big Thunder at 7:20 (due to the standby wait time) would not be denied access to a restroom between Big Thunder and the park main gate, but who knows. That just doesn't seem very "Disney" to me.