The drop site was found to be an old war era AA-gun captured from the soviets, an ItK 37 "Tuulimäki", a 76mm AA gun located in Tuulimäki, Espoo, just by the Tapiola office.
The memorial is for the AA battery 32 "Länsi" (West), the battery had 4 of the same model on Tykkimäki at Tapiola during 1942-1944. The AA-gun is just a few meters from the public pedestrian road with street lights.
The drop was right under gunner's seat below the gun's baseplate and was not visible without trying to look for it or tap below the surface. The drop was a sealed clear bag duct taped to the bottom with a black bag inside it, sealed once from the both ends with a smaller seal to create 4 1 cm by 1 cm pockets on the ends. The contents are what appears to be four rare-earth magnets, a 5cm long spring and a tube with an end where the spring fits snuggly.

The contents of the bag: http://i.imgur.com/MldGC.jpg
Closer look on the tube: http://i.imgur.com/LOwUN.jpg http://i.imgur.com/LOwUN.jpg
Sealed Black Bag: http://i.imgur.com/6UKKP.jpg http://i.imgur.com/MRqtp.jpg
The drop with the outer clear sealed bag: http://i.imgur.com/yALCf.jpg
Where it was found: http://i.imgur.com/TJd70.jpg http://i.imgur.com/O8wKb.jpg http://i.imgur.com/CrOBo.jpg
The contents along a cm-scale ruler: http://i.imgur.com/1jH4a.jpg http://i.imgur.com/MldGC.jpg