Definition: always watchful
Usage: to an eye like mine, a lidless watcher of the public weal- Alfred Tennyson
Similar words: alert, watchful
Definition: engaged in or accustomed to close observation
Usage: caught by a couple of alert cops; alert enough to spot the opportunity when it came; constantly alert and vigilant, like a sentinel on duty
Synonyms: lidless
Definition: having no lid
Usage: a lidless container