Thank you Beckles and alb - the San Juan (PR) club "sells" himself as American Express Lounge, it is located just after the gate UA uses normally for its ORD-flights. I had a UA-voucher to use it - but asked the lady about American Express member access - and she replied, anybody with a gold-card like mine (there are no AEx platinums in Switzerland) could use it free - but she didn't know anything about other locations.