This document describes how to improve the security on your Chromebook by forcing the password field to appear after opening the lid or waking from sleep mode. This function is not enabled by default and adds a considerable amount of security.
You are then prompted for your password. Switch on the “Show lock screen when waking from sleep” setting.
After this setting is turned on, your screen will prompt you for your password after it is awakened from sleep mode (or closing the lid).