GVfs trash implementation creates file monitors for all mount points. It doesn't care about access permissions. Monitor implementation polls the files every about 4 seconds if there aren't sufficient access permissions.
AutoFS mounts are ordinarily unmounted if they aren't using for some time, however file monitor prevents AutoFS mounts from being expired. It causes also high system load if there is lot of AutoFS mounts.
Mount points without read access permissions aren't longer monitored.
File monitors aren't polling mount points without read access, therefore also AutoFS mounts can expire.