"vigilant" in the adjective sense
1. argus-eyed, open-eyed, vigilant, wakeful
carefully observant or attentive on the lookout for possible danger
"a policy of open-eyed awareness"
"the vigilant eye of the town watch"
"there was a watchful dignity in the room"
"a watchful parent with a toddler in tow"
Source: WordNet® (An amazing lexical database of English)
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