Once upon a time in Umag there was the Casino Solei, if you are interested in its history the Internet provides a lot of Information.
The Header Coordinates mark the entrance. Go in and start your search
Your task is to find the room shown in the Spoiler Pic, the blue cross marks the Hideout.
Take care of the risks a Lost Place provides and take a torch with you. The floor can be tricky sometimes.
Hope you are lucky in suceeding😃
Ps: photos of this lost place posted would be nice