Desktop unlocked on secondary monitor

[zulis]

Using Linux Mint Cinnamon 18.2 on Lenovo Y700.

Linux tadas-linux 4.10.0-37-generic #41~16.04.1-Ubuntu SMP Fri Oct 6 22:42:59 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

No LSB modules are available.
Distributor ID: LinuxMint
Description: Linux Mint 18.2 Sonya
Release: 18.2
Codename: sonya

Steps to reproduce:
1. Connect secondary monitor. This secondary one is used as main monitor.
2. Disconnect all cables, including power, from the laptop. Do some actions on laptop ex. move some windows.
3. Lock the screen on laptop using ctrl+alt+l
4. Move mouse using track pad on laptop, until password input box appears.
4. Connect all laptop cables.
5. On laptop screen is locked, but secondary monitor wakes up and shows desktop. I can do whatever I want on this external monitor.

This happens every time I take my laptop to the meeting and bringing it back.