Since the guard has left his post, go there to investigate. However, first go to the spot where his bike was parked, and pick up the brick in zoom. Go to the checkpoint, enter the terrace and go around it - there are glass doors on the river side of the building. They're obviously closed, just like those on the opposite side, but you can utilize the brick to "open" it.
Get into the next room and collect the papers lying on the table (customs officer's pile of papers).
When getting back, zoom in on a wicker basket and take the hip flask (customs officer's flask).
Get outside and go to the left on the terrace, get to the ladder and get down. Then go right and enter another room in the building - it's a sort of a magazine/cellar.
Press the red button on the beam supporting the ceiling (vis-a-vis the door). From the opposite side of the river, a Tyrolean will arrive - a metal basket traversed on a rope. Put the flask inside, enter the building again and press the button. The basket will depart and presently come back with a gift from the watcher - zoom in and take from it bags of sawdust. Get back up on the ladder.