The first secret on this map is through the double button door. The closed door in there can be opened with the other button above the steam. In the door there is a med pack that can reheal any of your teammates. Here are screens to show.
Door within the double buttoned door:
The other button above the steam:
The box that gives you a medpack:
Secret two is just after secret one and it is up by the window. You should kill the helicopter before you do it but i could not be bothered :P. Here are the screens (It is not that good).
The text says "I know your game you cheeky b******!"
Secret three is quite a while after the second one. It is after you get to the fan room with the 3 buttons. Here are the screens (Another not so good one).
That concludes the secrets for Area49.