I have tested this with both the 6.x-1.0 and 6.x-dev versions for Signup and both worked the same way.
When a user signs up for a node and they are the last user, causing the signup to be closed for that node, it does not clear the cache.
Caching turned on for 1 hour.
User signs up for node.
Signups go to a closed status.
Confirmation messages/signup closed messages display to user.
A new user goes to the same node.
Signup form is displayed to the user.
User can signup.
No messages are displayed.