Shoot it, revealing a switch:
Pull the switch:
and a secret door opens on the wall opposite:
allowing access to a secret room:
But hey - we can climb this drainpipe:
and then jump onto the window box before entering the sniper's room. Oh, looks like he's asleep:
The room itself is a secret area:
As an added bonus, Dr Merkwurdigliebe will now go for his evening walk. If only there was some way we could show him our appreciation of his nice velvet smoking jacket:
Shoot the padlock off, and go through to the ladder:
go down the ladder, walk along the passage, and up a second ladder:
which takes is into Dr Merkwurdigliebe's backroom.
(note: if you solve secret three before secret two, the good Doctor will be working in his lab).
The bung on the left barrel is suspicious - push it:
this makes a secret door open in the wall to the left of the barrels:
go through to enter a secret chamber:
Kick open the box, to reveal some treasure:
Notice - the lingerie lady is no longer cowering in fear, but is much more friendly. Ladies like men who're good at kicking stuff, apparently.