Six flats in two weeks

I've had more flat tyres in the last two weeks than in the last 12 months. The weird thing is that after removing the tubes on some of those occasions, pumping them up and putting them through a bucket of water, I haven't been able to detect any air leaks - not a single bubble.

However, next morning, the tyre is flat as a pancake.

I've tried new tyres and new tubes - no dice. I wonder if I've annoyed somebody at work, and they're sneaking down to the garage to deflate my tyres during the day....