Me and the Mrs went out with a few friends in London one evening. After drinks at one house we moved to another house to pick up a couple more people before going to the pub. Whilst there and having a drink I needed a pee and as there were quite a few of us there someone was already in the toilet and taking ages. One of the girls who lived there (mid 20s slim, fit, with boyfriend also there) suggested I go up stairs and use her en-suite, save me waiting. I went on up, shut the door, went for a pee and spotted the laundry basket with the lid off and some dirty washing on the floor. Looked as if she'd finished work, come home and got changed quickly. I was a bit drunk and felt the sudden urge to rifle through and find her knickers. I found them a d gave then a sniff and they were lovely and fresh. I bashed out a quick wank there and then and was back downstairs before anyone had even noticed.
It wasn't til the next day I suddenly realised what I'd actually done and was paranoid there might have been a camera in there and I'd be outed to everyone as a panty sniffer!
Not done it since, but the opportunity has never really come up since.